Baxi 400 Combi (range) Fault codes & diagnostics

129 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 24-28 kW 2015-present

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 111 High 8 Medium 8 Low
Just want the most common Baxi 400 Combi (range) faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 129 documented codes

Tap any card for details

LED / flash-pattern codes
1 green flash

Your boiler has reached its set temperature and is temporarily resting; it is working exactly as it should.

Low DIY-safe

What to check first

  1. No action is required.
  2. Wait for the central heating or hot water demand to continue; the light will change once the temperature drops.
  3. Ensure your thermostat is set to the desired temperature if you feel the house is too cold.

Related codes

Full explanation for 1 green flash
1 red flash

Your boiler has detected that it is getting too hot or that water isn't moving through the system correctly, causing it to shut down for safety.

High DIY-safe £120-280

What to check first

  1. Check that your central heating valves and radiator valves are open
  2. Check the boiler pressure gauge and top up to 1.5 bar if it is too low
  3. Turn the selector switch to the reset position for five seconds, then back to the original setting

Parts commonly replaced

  • Temperature Sensor (NTC Thermistor)
  • Circulating Pump
  • Overheat Thermostat

Related codes

Full explanation for 1 red flash
2 red flashes

Your boiler has become too hot and has switched itself off as a safety precaution to prevent damage.

High DIY-safe £100-£350

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Ensure there is enough water pressure in the system (between 1.0 and 1.5 bar).
  3. Turn the selector knob to the reset position for five seconds and then back to its original setting.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for 2 red flashes
3 green flashes

Your boiler is struggling to keep its flame lit and is currently trying to fix itself by restarting.

Medium DIY-safe £80-250

What to check first

  1. Check that your gas supply is active and other gas appliances are working
  2. Monitor the boiler to see if it self-resets and stays lit
  3. Ensure the external flue terminal is not obstructed by debris
  4. If the fault persists or turns to red flashes, contact a Gas Safe engineer

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve

Related codes

Full explanation for 3 green flashes
3 red flashes

Your boiler has tried to light itself several times but has been unable to sustain a flame, leaving you without heating or hot water.

High DIY-safe £100-£280

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property
  2. If it is freezing outside, check that the plastic condensate pipe leading outdoors is not frozen and blocked
  3. Press and hold the reset button for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 3 red flashes
4 green flashes

Your boiler is temporarily struggling to talk to its internal computer and should automatically fix itself and restart shortly.

Low DIY-safe £0-120

What to check first

  1. Wait up to 10 minutes for the boiler to self-reset
  2. If the error persists, turn the selector switch to the reset position for 5 seconds
  3. Check that any external room thermostats have fresh batteries

Parts commonly replaced

  • None (usually self-clears)
  • Main PCB (if fault recurs)

Related codes

Full explanation for 4 green flashes
4 red flashes

Your boiler has stopped working because the internal fan, which safely clears waste gases away, is not running correctly.

High Engineer only £200-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Air pressure switch

Related codes

Full explanation for 4 red flashes
5 green flashes

Your boiler has experienced a temporary electronic glitch in its internal settings and is currently attempting to automatically reset itself.

Low DIY-safe £0-350

What to check first

  1. Wait for the boiler to complete its automatic self-reset process
  2. If the flashing continues for more than 10 minutes, turn the power off and back on at the fused spur switch
  3. Monitor the boiler to see if the error recurs frequently

Parts commonly replaced

  • None (if temporary)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 5 green flashes
5 red flashes

Your boiler's internal memory chip has failed, meaning the main control board can no longer read its essential settings.

High Engineer only £180-350

Parts commonly replaced

  • Parameter Storage Unit (PSU)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 5 red flashes
6 green flashes

Your boiler has detected a minor temporary glitch and is waiting a moment before naturally resuming normal operation on its own.

Low DIY-safe

What to check first

  1. Wait 5-10 minutes for the boiler to automatically clear the error
  2. Ensure your electricity supply is stable and not fluctuating
  3. If the flashing continues, try a simple reset by turning the selector knob to 'R' for five seconds

Related codes

Full explanation for 6 green flashes
Showing 11–20 of 129
6 red flashes

Your boiler has encountered an internal timing or communication fault and has likely locked itself out for safety.

High DIY-safe £100-350

What to check first

  1. Locate the selector switch on the front of the boiler
  2. Turn the switch to the 'R' (Reset) position and hold it for at least 5 seconds
  3. Return the switch to your desired setting (Heating/Hot Water) and wait to see if the error clears

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Loom

Related codes

Full explanation for 6 red flashes
FLASH FLASH ON

Your boiler has detected an issue with the internal fan that clears waste gases, preventing it from starting up for safety reasons.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Air Pressure Switch

Related codes

Full explanation for FLASH FLASH ON
FLASH OFF FLASH

Your boiler is not receiving enough electrical power from the mains supply to operate safely.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Voltage Regulator

Related codes

Full explanation for FLASH OFF FLASH
FLASH OFF ON

Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £120-350

What to check first

  1. Wait 30 minutes for the system to cool down completely.
  2. Check that your radiator valves are open and not turned to '0'.
  3. Turn the selector switch to the 'R' (Reset) position for five seconds, then turn it back to your desired setting.

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • Heat Exchanger

Related codes

Full explanation for FLASH OFF ON
FLASH ON FLASH

Your boiler's internal computer has encountered a critical error and needs a professional to reset or replace it.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for FLASH ON FLASH
FLASH ON ON

Your boiler's internal computer has developed a fault and is unable to control the system safely.

High Engineer only £280-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for FLASH ON ON
ON FLASH FLASH

Your boiler has stopped heating because water is not moving through the system properly or there is not enough water inside.

High DIY-safe £100-350

What to check first

  1. Check the pressure gauge on the front of the boiler is between 1 and 1.5 bar
  2. If the pressure is low, use the filling loop underneath to top up the system
  3. Ensure all radiator valves and the boiler isolation valves are fully open
  4. Reset the boiler by turning the selector knob to 'R' for five seconds

Parts commonly replaced

  • Circulation Pump
  • Main Heat Exchanger
  • Water Pressure Sensor

Related codes

Full explanation for ON FLASH FLASH
ON FLASH ON

Your boiler has detected a temperature sensor fault, meaning it can't accurately measure how hot the water is getting and has stopped working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating NTC Sensor
  • Domestic Hot Water NTC Sensor
  • Wiring harness

Related codes

Full explanation for ON FLASH ON
ON OFF FLASH

Your boiler has detected that the electrical wiring coming into the unit from your house has been connected incorrectly, swapping the live and neutral wires.

High Engineer only £80-160

Parts commonly replaced

  • External wiring
  • Fused spur
  • Control PCB

Related codes

Full explanation for ON OFF FLASH
ON ON FLASH

Your boiler's internal computer has encountered a critical electronic failure and can no longer control the heating and hot water systems safely.

High Engineer only £250-£450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for ON ON FLASH
Digital codes
110

Your boiler has switched itself off because it has gotten too hot, likely due to a blockage or air trapped in the system stopping the water from circulating.

High DIY-safe £100-280

What to check first

  1. Turn off the boiler and allow the system to cool down completely.
  2. Bleed all radiators using a radiator key to release trapped air.
  3. Check the boiler pressure gauge and top up to 1.5 bar if it has dropped.
  4. Press the Reset button on the front of the boiler for five seconds.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Temperature Sensor (NTC)

Related codes

Full explanation for 110
117

Your boiler has too much water in the system, which puts unnecessary stress on the pipes and internal components.

Medium DIY-safe £0-120

What to check first

  1. Check that the filling loop valves are tightly closed
  2. Bleed water from a radiator using a radiator key to reduce pressure
  3. Monitor the pressure gauge until it returns to the green zone (typically 1.0 to 1.5 bar)
  4. Reset the boiler once the pressure is corrected

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve (PRV)
  • Expansion vessel

Related codes

Full explanation for 117
118

Your boiler has detected that the water pressure is too low for it to operate safely, which usually means it needs a simple top-up of water.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath the boiler.
  2. Open the one or two taps/levers on the filling loop slowly until you hear water moving.
  3. Watch the pressure gauge and close the taps once it reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for 118
119

Your boiler has detected that the water pressure in the heating system is too low to operate safely, which has caused it to shut down.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Slowly open the one or two taps on the filling loop until you hear water entering.
  3. Watch the pressure gauge and close the taps once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Pressure sensor
  • Filling loop valve
  • Expansion vessel repressurisation

Related codes

Full explanation for 119
125

Your boiler has stopped working because the water isn't moving through the system properly, which is causing it to overheat and shut down for safety.

High DIY-safe £150-£350

What to check first

  1. Check if your radiator valves are open
  2. Ensure your boiler pressure is between 1.0 and 1.5 bar on the gauge
  3. Bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulation Pump
  • Pump Lead
  • Main Heat Exchanger (if scaled up)

Related codes

Full explanation for 125
127

Your boiler has failed to heat the water to a high enough temperature to complete its automatic hygiene cycle for killing bacteria.

Low DIY-safe £0-120

What to check first

  1. Check if the hot water temperature is set too low on the boiler control panel
  2. Ensure there has not been a prolonged period of high hot water demand during the cycle
  3. Press the reset button to allow the boiler to re-attempt the hygiene cycle
  4. Monitor for a recurrence; if the code returns, contact a professional

Parts commonly replaced

  • Hot water temperature sensor (NTC)
  • Diverter valve
  • Plate heat exchanger

Related codes

Full explanation for 127
128

Your boiler started to heat up but the flame went out unexpectedly while it was running.

High DIY-safe £100-£250

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm gas supply.
  2. If it is freezing outside, check if your external condensate pipe is frozen and thaw it with warm water.
  3. Reset the boiler by turning the selector switch to 'R' for five seconds and then back to the original position.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for 128
133

Your boiler has tried to light several times but cannot establish a flame, which means your heating and hot water will not work.

High DIY-safe £100-350

What to check first

  1. Check if your gas prepay meter has credit
  2. Check that other gas appliances, like a hob, are working to confirm gas supply
  3. Look at the condensate pipe outside to see if it is frozen or blocked
  4. Press and hold the Reset button for 5 seconds

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 133
153

Your boiler has been manually reset or had its lock released, but it may have hit the maximum number of reset attempts allowed by the system.

High DIY-safe £80-120

What to check first

  1. Locate the reset button or selector switch on the front control panel.
  2. Press and hold the reset button for at least five seconds.
  3. Wait for the boiler to cycle through its startup sequence.

Parts commonly replaced

  • No parts usually required
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 153
160

Your boiler has detected a problem with its internal fan, which is preventing it from safely clearing away exhaust fumes and starting the heating process.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 160
Showing 31–40 of 129
162

Your boiler has detected an issue with how it clears exhaust fumes, causing it to shut down for safety.

High Engineer only £100-280

Parts commonly replaced

  • Air pressure switch
  • Venturi tube
  • Flue fan

Related codes

Full explanation for 162
164

Your boiler has detected that water is not circulating correctly or the sensor responsible for checking water flow is faulty, meaning it won't heat your home or water for safety reasons.

High Engineer only £120-190

Parts commonly replaced

  • Pressure Sensor
  • Flow Switch
  • Wiring Harness

Related codes

Full explanation for 164
166

Your boiler has stopped working because it cannot confirm that the fan is correctly clearing exhaust gases from the system.

High Engineer only £120-£260

Parts commonly replaced

  • Air Pressure Switch
  • Venturi Tube
  • Fan

Related codes

Full explanation for 166
270

Your boiler is overheating because water isn't moving through the system properly or there isn't enough water inside.

High DIY-safe £120-350

What to check first

  1. Check the pressure gauge and top it up to 1.5 bar if it is too low
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Main Heat Exchanger (if scaled)

Related codes

Full explanation for 270
432

Your boiler has detected an issue with its electrical grounding or wiring connections, which is preventing it from running safely.

High Engineer only £100-250

Parts commonly replaced

  • Earth connection lead
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 432
433

Your boiler has become too hot and has shut down as a safety precaution to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate
  2. Ensure there is enough water pressure in the system by checking the gauge
  3. Reset the boiler by turning the selector knob to 'R' for five seconds

Parts commonly replaced

  • Central heating pump
  • Plate heat exchanger
  • NTC Temperature sensor

Related codes

Full explanation for 433
E.00 – .04

Your boiler has detected that a temperature sensor is disconnected or faulty, which means the system cannot monitor internal water heat levels safely.

High Engineer only £90-160

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Wiring harness/connector

Related codes

Full explanation for E.00 – .04
E.00 – .05

Your boiler has detected a problem with the sensor that monitors internal water temperature, preventing it from heating up correctly for your safety.

High Engineer only £90-160

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Wiring harness or connector
Full explanation for E.00 – .05
E.01 – .02

Your boiler has detected that the water coming back from your radiators is hotter than the water leaving the boiler, which usually means there is a sensor error or a circulation problem.

High DIY-safe £100-250

What to check first

  1. Locate the reset button or selector switch on the front of the boiler control panel.
  2. Press or turn the control to the 'R' (Reset) position for 5 seconds.
  3. If the fault persists, check that all radiator valves are fully open and the system pressure is between 1 and 1.5 bar.

Parts commonly replaced

  • NTC Temperature Sensor
  • Circulation Pump
  • Main PCB

Related codes

Full explanation for E.01 – .02
E.01 – .04

Your boiler is struggling to keep its flame lit and has repeatedly shut down for safety because it cannot maintain a steady burn.

High DIY-safe £120-350

What to check first

  1. Check that your gas meter has credit and the gas supply valve is open
  2. Inspect the outdoor condensate pipe for signs of freezing or blockages
  3. Press and hold the 'R' or Reset button for 5 seconds to restart the system

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • PCB

Related codes

Full explanation for E.01 – .04
Showing 41–50 of 129
E.01 – .11

Your boiler's fan is not spinning at the correct speed to safely clear waste gases, often caused by a strong gust of wind or a mechanical fault.

High DIY-safe £200-450

What to check first

  1. Check if it is a very windy day and wait for the weather to calm down
  2. Identify the flue pipe outside and ensure it isn't blocked by leaves or debris
  3. Press the Reset button for five seconds

Parts commonly replaced

  • Fan Assembly
  • Air Pressure Switch
  • Main PCB

Related codes

Full explanation for E.01 – .11
E.01 – .12

Your boiler has sensed that the water returning from your radiators is hotter than the water leaving the boiler, which usually means the water isn't circulating correctly or a sensor has developed a fault.

High DIY-safe £90-180

What to check first

  1. Press and hold the Reset button for 5 seconds.
  2. Ensure all radiator valves (TRVs) are turned to their highest setting to allow water to flow.
  3. Check that the boiler pressure gauge is between 1 and 1.5 bar, and top it up using the filling loop if it is too low.

Parts commonly replaced

  • Temperature Sensor (NTC thermister)
  • Circulation Pump
  • Return Water Pipe Sensor

Related codes

Full explanation for E.01 – .12
E.01 – .17

Your boiler has stopped working because water isn't moving through the system correctly or a temperature sensor has lost its connection.

High DIY-safe £90-220

What to check first

  1. Press and hold the 'R' or 'Reset' button for five seconds
  2. Check that your radiator valves and isolation valves under the boiler are fully open
  3. Ensure your system pressure is topped up to between 1 and 1.5 bar

Parts commonly replaced

  • Temperature sensor (NTC thermistor)
  • Wiring harness
  • Circulation pump

Related codes

Full explanation for E.01 – .17
E.01 – .20

Your boiler has switched itself off because it is getting too hot inside, likely due to a lack of water flow or a faulty sensor.

High DIY-safe £120-300

What to check first

  1. Check if your heating system pressure has dropped and top it up to 1.5 bar if needed
  2. Ensure all radiator valves are fully open to allow water to circulate
  3. Reset the boiler by turning the selector switch to 'R' for five seconds

Parts commonly replaced

  • Flue Thermostat (NTC Sensor)
  • Circulation Pump
  • Main Heat Exchanger (if scaled)

Related codes

Full explanation for E.01 – .20
E.02 – .00

Your boiler is simply notifying you that the reset button has been pressed and it is restarting itself.

Low DIY-safe

What to check first

  1. Wait approximately 30 seconds for the boiler to complete its startup sequence.
  2. The code should disappear automatically once the boiler resumes normal operation.
  3. If the code persists or changes, check if there is an underlying fault that caused you to press reset initially.

Related codes

Full explanation for E.02 – .00
E.02 – .07

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with small taps under the boiler).
  2. Slowly open both taps/levers to allow water into the system.
  3. Watch the pressure gauge until it reaches between 1 and 1.5 bar, then close both taps tightly.
  4. Restart the boiler to clear the fault code.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for E.02 – .07
E.02 – .16

Your boiler has experienced an internal electrical failure that has caused it to shut down for safety, and it cannot restart without professional repair.

High Engineer only £250-£450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Channel Service Unit

Related codes

Full explanation for E.02 – .16
E.02 – .17

Your boiler has developed a communication fault between its internal computer and the gas control unit, meaning it cannot safely manage the fuel it needs to fire up.

High Engineer only £180-£450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness
Full explanation for E.02 – .17
E.02 – .19

Your boiler's internal configuration settings have been altered or are incorrectly set, which is preventing it from running properly.

High Engineer only £80-160

Parts commonly replaced

  • Main PCB
  • Selector Link/Jumper
Full explanation for E.02 – .19
E.02 – .20

Your boiler's internal computer settings have been changed or are set incorrectly, meaning the control board doesn't know how to operate the unit safely.

High Engineer only £90-180

Parts commonly replaced

  • Main PCB
  • Internal wiring harness
Full explanation for E.02 – .20
Showing 51–60 of 129
E.02 – .21

Your boiler's internal settings have been configured incorrectly or a part has been replaced that hasn't been programmed yet, causing the system to stop working.

High Engineer only £80-160

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Selector Jumpers
Full explanation for E.02 – .21
E.02 – .47

Your boiler is having trouble communicating with your external heating controls, such as your thermostat or programmer, due to a wiring issue.

High Engineer only £90-180

Parts commonly replaced

  • Receiver unit
  • External thermostat
  • Control wiring bundle
Full explanation for E.02 – .47
E.02 – .48

Your boiler is having trouble communicating with your external thermostat or wireless controls, preventing your heating from turning on correctly.

Medium DIY-safe £80-160

What to check first

  1. Check that your wireless thermostat has fresh batteries installed
  2. Ensure the external control unit is powered on and within range of the boiler
  3. Try resetting the boiler and following the manufacturer's instructions to re-pair the device

Parts commonly replaced

  • Batteries
  • RF Receiver unit
  • Wireless Room Thermostat
Full explanation for E.02 – .48
E.04 – .00

Your boiler has lost its connection to the internal gas control system, meaning it cannot safely ignite to provide heating or hot water.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E.04 – .00
E.04 – .01

Your boiler has stopped working because it cannot correctly measure the temperature of the water leaving the unit due to a faulty or disconnected internal sensor.

High Engineer only £90-160

Parts commonly replaced

  • Flow Temperature Sensor (NTC Thermistor)
  • Wiring Harness

Related codes

Full explanation for E.04 – .01
E.04 – .02

Your boiler has stopped working because it cannot correctly measure the temperature of the water leaving the unit due to a sensor or wiring connection issue.

High Engineer only £90-160

Parts commonly replaced

  • Flow temperature NTC sensor
  • Sensor wiring harness
Full explanation for E.04 – .02
E.04 – .03

Your boiler has reached a critically high temperature and shut down because water is not moving around the system correctly.

High DIY-safe £180-450

What to check first

  1. Check that the boiler pressure gauge is between 1 and 1.5 bar
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Bleed all radiators to remove trapped air that could be blocking water flow

Parts commonly replaced

  • Circulation Pump
  • Flow Temperature Sensor (NTC)
  • Main Heat Exchanger (if scaled or blocked)

Related codes

Full explanation for E.04 – .03
E.04 – .04

Your boiler has detected a problem with the sensor that monitors exhaust gas temperatures, causing the system to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Flue Overheat Thermostat (NTC Sensor)
  • Wiring Harness
  • Main Control Board

Related codes

Full explanation for E.04 – .04
E.04 – .05

Your boiler has detected an issue with the sensor that monitors exhaust gases, and it has shut down as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Flue temperature sensor (NTC)
  • Wiring harness
  • Main PCB
Full explanation for E.04 – .05
E.04 – .06

Your boiler has shut down because it is getting too hot inside, likely due to a blockage in the air pipes or a failure in the system that moves water around.

High Engineer only £120-350

Parts commonly replaced

  • Flue Gas NTC Sensor
  • Circulation Pump
  • Main Heat Exchanger
Full explanation for E.04 – .06
Showing 61–70 of 129
E.04 – .08

Your boiler has shut down because it has detected it is getting too hot, likely due to water not moving through the system properly.

High DIY-safe £120-280

What to check first

  1. Check that your radiator valves are open to allow water to circulate.
  2. Ensure your boiler pressure is between 1.0 and 1.5 bar.
  3. Try resetting the boiler by turning the selector knob to 'R' for five seconds.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Temperature Sensor (NTC Slave)

Related codes

Full explanation for E.04 – .08
E.04 – .10

Your boiler is trying to start up but cannot light the gas flame needed to heat your water and radiators.

High DIY-safe £100-£250

What to check first

  1. Check if other gas appliances, like a hob, are working to confirm your gas supply is active.
  2. If you have a prepaid gas meter, ensure it has sufficient credit.
  3. Press and hold the 'Reset' button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for E.04 – .10
E.04 – .12

Your boiler has detected a flame when there shouldn't be one, or the internal burner is becoming dangerously hot.

Emergency Engineer only £120-450

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Printed Circuit Board (PCB)
  • Burner door insulation

Related codes

Full explanation for E.04 – .12
E.04 – .13

Your boiler's internal fan, which is essential for safely clearing exhaust fumes, has stopped spinning or cannot communicate with the main system.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)
Full explanation for E.04 – .13
E.04 – .17

Your boiler has detected a serious internal problem with how it manages gas flow or the combustion process, and has locked itself out for safety.

High Engineer only £220-450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Venturi/Air Unit

Related codes

Full explanation for E.04 – .17
E0

Your boiler's internal thermometer has stopped working correctly, so the system has shut down because it cannot accurately measure the water temperature.

High Engineer only £90-160

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for E0
E1

Your boiler has become too hot and has shut down as a safety precaution, often because air is trapped in your heating system preventing water from circulating.

High DIY-safe £80-220

What to check first

  1. Turn off the boiler and allow the radiators to cool down completely.
  2. Use a radiator key to bleed any trapped air from all radiators in your home.
  3. Check the boiler pressure gauge and use the filling loop to top it back up to 1.5 bar if it has dropped.
  4. Reset the boiler by turning the selector switch to 'R' for five seconds.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Automatic Air Vent

Related codes

Full explanation for E1
E2

Your boiler is struggling to move water around the system, usually because of trapped air or a blockage.

High DIY-safe £80-250

What to check first

  1. Turn off your heating and let the radiators cool down.
  2. Use a radiator key to bleed any trapped air from all radiators.
  3. Check your boiler pressure gauge and top it up to 1.5 bar if it has dropped.

Parts commonly replaced

  • Circulation Pump
  • Auto Air Vent
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for E2
E3

Your boiler's internal computer has encountered a critical error and can no longer control the heating system safely.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E3
E4

Your boiler is struggling to light its flame, which might be caused by air trapped in your heating system preventing the water from flowing correctly.

High DIY-safe £100-280

What to check first

  1. Check that your gas pre-payment meter (if applicable) has credit
  2. Switch the boiler off and back on again to attempt a reset
  3. Bleed your radiators with a radiator key to release any trapped air
  4. Ensure your boiler pressure is between 1.0 and 1.5 bar on the gauge

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Generator

Related codes

Full explanation for E4
Showing 71–80 of 129
E09

Your boiler has stopped working because it has lost communication with the internal gas valve, meaning it cannot light the flame to provide heating or hot water.

High Engineer only £120-280

Parts commonly replaced

  • Gas valve wiring harness
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E09
E10

Your boiler is having trouble communicating with the optional sensor used to measure the temperature outside, meaning it might not heat your home efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • External temperature sensor
  • Wiring loom
  • Main PCB

Related codes

Full explanation for E10
E12

Your boiler has detected that water is not circulating properly through the system, which is preventing it from heating up safely.

High DIY-safe £120-280

What to check first

  1. Check that the boiler pressure is between 1.0 and 1.5 bar on the pressure gauge
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Bleed your radiators to remove any trapped air that might be blocking water flow

Parts commonly replaced

  • Circulation pump
  • Differential pressure sensor
  • Flow switch

Related codes

Full explanation for E12
E13

Your boiler has detected that a internal water flow sensor is stuck in the 'on' position even when the pump is not running.

High Engineer only £120-220

Parts commonly replaced

  • Differential pressure switch
  • Microswitch
  • Hydraulic venturi block

Related codes

Full explanation for E13
E15

Your boiler has detected an internal electrical communication error with the gas control system and has shut down for safety.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E15
E20

Your boiler's internal temperature sensor has developed a fault or is disconnected, meaning the system cannot safely monitor how hot the water is for your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating NTC Sensor
  • Thermistor
  • Wiring Harness

Related codes

Full explanation for E20
E28

Your boiler has detected an issue with how it releases exhaust gases or a possible blockage, causing the system to shut down for safety.

High Engineer only £120-£280

Parts commonly replaced

  • Flue NTC Sensor
  • Air Pressure Switch
  • Condensate Trap

Related codes

Full explanation for E28
E40

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating Return NTC Sensor
  • Sensor Wiring Harness
  • Control Board

Related codes

Full explanation for E40
E50

Your boiler has detected a problem with the sensor that monitors the temperature of your hot water, meaning your taps may only run cold.

High Engineer only £90-160

Parts commonly replaced

  • Hot water NTC thermistor
  • Wiring harness

Related codes

Full explanation for E50
E53

Your boiler has detected an issue with the air supply or how the gas is burning, meaning it cannot operate safely and has shut down.

Emergency Engineer only £120-350

Parts commonly replaced

  • Gas Valve
  • PCB (Printed Circuit Board)
  • Fan
  • Flue Seals

Related codes

Full explanation for E53
Showing 81–90 of 129
E54

Your boiler is having trouble detecting when you turn on your taps, which means it might not provide hot water when you need it.

Medium Engineer only £120-190

Parts commonly replaced

  • DHW Flow Sensor
  • Flow Turbine
  • Wiring Harness

Related codes

Full explanation for E54
E55

Your boiler’s internal computer has lost track of its fine-tuned settings and needs to be recalibrated to ensure the gas is burning efficiently.

High Engineer only £80-180

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Electrode

Related codes

Full explanation for E55
E78

Your boiler has lost its ability to accurately monitor the internal water pressure, causing the system to shut down as a safety precaution.

High Engineer only £140-£220

Parts commonly replaced

  • Water Pressure Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for E78
E83/E84/E85/E86/E87

Your boiler's internal computer has lost contact with its control sensors or external thermostat, meaning it cannot receive the signals needed to fire up.

High Engineer only £120-£350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness
  • External Controls Receiver

Related codes

Full explanation for E83/E84/E85/E86/E87
E92

Your boiler has detected an issue with the fuel and air mixture while trying to automatically calibrate itself.

High Engineer only £100-£350

Parts commonly replaced

  • Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for E92
E100

Your boiler's internal computer has lost track of the correct time or date, which is preventing its scheduled operations from working correctly.

Medium DIY-safe £0-120

What to check first

  1. Locate the user controls on the front of the boiler panel
  2. Navigate to the clock/time settings menu
  3. Update the time and date to the current values and save the settings
  4. Press the Reset button once to clear the fault code

Parts commonly replaced

  • None (Software reset)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E100
E109

Your boiler has detected that hot water isn't moving through the system properly, which causes it to shut down to prevent overheating.

High DIY-safe £120-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the internal gauge.
  2. Ensure all radiator valves are fully open and not turned to the off position.
  3. Gently bleed your radiators using a radiator key to remove trapped air.

Parts commonly replaced

  • Circulation Pump
  • Main Heat Exchanger
  • Flow/Return Sensor

Related codes

Full explanation for E109
E110

Your boiler has shut down because it has reached an excessively high temperature, triggering a safety switch to prevent damage.

High DIY-safe £100-280

What to check first

  1. Wait 30 minutes for the boiler to cool down completely.
  2. Ensure all radiator valves (TRVs) are fully open and there are no blockages.
  3. Press and hold the 'R' or Reset button for five seconds to restart the system.

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • Limit Stat

Related codes

Full explanation for E110
E117

Your boiler has detected that there is too much water pressure in the system, which can cause internal leaks if not addressed.

Medium DIY-safe £0-120

What to check first

  1. Check that the filling loop valves are fully closed
  2. Bleed water from a radiator using a bleed key to reduce pressure
  3. Monitor the pressure gauge until it returns to between 1.0 and 1.5 bar
  4. Reset the boiler once the pressure is correct

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve
  • Expansion vessel

Related codes

Full explanation for E117
E118

Your boiler has stopped working because the water pressure inside the system has dropped too low and needs to be topped up.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath your boiler.
  2. Open both taps on the filling loop slowly until you hear water flowing.
  3. Watch the pressure gauge and close both taps once the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for E118
Showing 91–100 of 129
E119

Your boiler has stopped working because the water pressure is too low for the system to run safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the silver flexible hose) underneath the boiler.
  2. Open the one or two taps on the loop slowly until you hear water flowing and the pressure gauge reaches 1.5 bar.
  3. Close the taps tightly and restart the boiler using the reset button.

Parts commonly replaced

  • Water pressure switch
  • Expansion vessel
  • Filling loop

Related codes

Full explanation for E119
E125

Your boiler has detected that hot water isn't moving through the system properly, which usually means the pump is stuck or there is an airlock blocking the flow.

High DIY-safe £150-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Bleed your radiators to remove any trapped air
  3. Ensure the boiler pressure is between 1.0 and 1.5 bar
  4. Reset the boiler to see if the fault clears

Parts commonly replaced

  • Circulation pump
  • Automatic air vent
  • Main heat exchanger

Related codes

Full explanation for E125
E128

Your boiler has tried and failed to stay lit too many times, so it has locked itself out for safety.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve is fully open (the yellow handle tucked under the boiler or by the meter).
  2. If you have a prepay meter, check that you have sufficient credit.
  3. Press the Reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Flame sensing electrode
  • Spark generator
  • Gas valve

Related codes

Full explanation for E128
E129

Your boiler is struggling to keep a steady flame alight while it tries to set itself up, causing it to shut down for safety.

High Engineer only £100-£250

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve

Related codes

Full explanation for E129
E130

Your boiler has detected that the exhaust gases are becoming too hot, causing the system to shut down as a safety precaution.

High Engineer only £90-180

Parts commonly replaced

  • Flue NTC Sensor
  • Heat Exchanger (cleaning/descaling)
  • Main PCB

Related codes

Full explanation for E130
E131

Your boiler has shut down because the exhaust gases have become too hot, which is a safety measure to prevent damage to the system.

High Engineer only £120-350

Parts commonly replaced

  • Heat Exchanger
  • Flue Thermistor
  • Burner Insulation Panel

Related codes

Full explanation for E131
E133

Your boiler is not receiving enough gas or is struggling to light the flame needed to heat your water and radiators.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your meter.
  2. If it is very cold outside, check if your white plastic condensate pipe has frozen and thaw it with warm water.
  3. Restart the boiler by turning the selector switch to the reset position for five seconds.

Parts commonly replaced

  • Gas Valve
  • Ignition Lead
  • Spark Electrode
  • PCB

Related codes

Full explanation for E133
E134

Your boiler has failed to ignite the gas after several attempts, typically because the gas supply is interrupted or the ignition system cannot detect a flame.

High DIY-safe £120-280

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Verify that your external gas lever/stopcock is in the 'on' position
  3. Check if other gas appliances, like a hob, are working to confirm gas supply
  4. Press and hold the 'R' or 'Reset' button for 5 seconds

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Spark Generator
  • Flame Sensing Probe

Related codes

Full explanation for E134
E135

Your boiler has stopped working because it cannot detect a steady flow of gas, which is likely due to an internal fault or a supply issue.

High DIY-safe £120-350

What to check first

  1. Check if other gas appliances, like a cooker, are working to confirm your gas supply is active.
  2. Ensure your gas meter has credit if you use a pre-payment system.
  3. Hold the Reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Ignition Electrode

Related codes

Full explanation for E135
E154

Your boiler has detected that water is circulting poorly or heating up too quickly, causing the temperature sensors to report an unusual reading.

High Engineer only £120-280

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Circulation pump

Related codes

Full explanation for E154
Showing 101–110 of 129
E160

Your boiler has stopped working because the internal fan, which safely exhausts fumes, is not spinning or communicating correctly with the main control board.

High Engineer only £180-£350

Parts commonly replaced

  • Extraction fan
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E160
E160 / E161

Your boiler has detected a problem with the fan that clears exhaust gases, preventing it from starting up for safety reasons.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for E160 / E161
E163

Your boiler's temperature sensor for hot water has developed a fault, meaning it cannot correctly measure or provide hot water to your taps.

Medium Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Connecting Wiring Loom

Related codes

Full explanation for E163
E164

Your boiler has detected an electrical issue with the device that monitors water pressure, meaning it cannot confirm if the system is safe to fire up.

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • PCB

Related codes

Full explanation for E164
E167

Your boiler’s internal control board has developed an electronic fault, meaning the computer that runs the system is no longer communicating correctly.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E167
E168

Your boiler's internal computer has encountered a memory or signal error and has locked itself down for safety.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • External Surge Protector

Related codes

Full explanation for E168
E193

Your boiler has detected that water is struggle to move through the system properly before the burner ignites, often due to a blockage or a trapped air pocket.

High DIY-safe £120-£350

What to check first

  1. Check that your boiler pressure is between 1.0 and 1.5 bar on the gauge
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Gently bleed your radiators to release any trapped air from the system

Parts commonly replaced

  • Circulation Pump
  • Automatic Air Vent
  • Main Heat Exchanger

Related codes

Full explanation for E193
E270

Your boiler has detected that water is failing to move through the system properly, causing it to overheat quickly and shut down for safety.

High DIY-safe £120-£350

What to check first

  1. Check if your radiator valves are fully open
  2. Check the boiler pressure gauge and top it up to 1.5 bar if it is too low
  3. Bleed all radiators to remove trapped air
  4. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation Pump
  • Temperature Sensor (NTC)
  • Main Heat Exchanger

Related codes

Full explanation for E270
E317

Your boiler has detected an issue with the electrical power timing coming into the unit, meaning the internal computer cannot sync with the mains supply.

High Engineer only £100-£350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Mains voltage stabilizer

Related codes

Full explanation for E317
E321

Your boiler has detected a problem with the sensor that monitors hot water temperature, which means your taps may only run cold or have inconsistent heat.

Medium Engineer only £90-£160

Parts commonly replaced

  • DHW NTC thermistor
  • Connecting wiring harness
Full explanation for E321
Showing 111–120 of 129
E384

Your boiler thinks a flame is burning even when it hasn't been asked to light, so it has shut down for safety purposes.

High Engineer only £120-300

Parts commonly replaced

  • Flame sensing electrode
  • Printed Circuit Board (PCB)
  • Gas valve

Related codes

Full explanation for E384
E385

Your boiler is not receiving enough electrical power to operate safely, which is usually caused by an external power cut or a failing internal control board.

High Engineer only £150-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Voltage Regulator

Related codes

Full explanation for E385
E430

Your boiler is checking if there is enough water pressure to run safely, which often means the system pressure has dropped too low.

High DIY-safe £0-120

What to check first

  1. Check the pressure gauge on the front of the boiler
  2. If the needle is below 1, use the filling loop handles underneath the boiler to top it up to 1.5 bar
  3. Reset the boiler by turning the selector switch to 'R' for five seconds

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for E430
H.01 – .00

Your boiler has stopped working because the internal control board can no longer talk to the gas valve, meaning the boiler cannot safely light the flame.

High Engineer only £180-450

Parts commonly replaced

  • Gas valve wiring harness
  • Gas valve
  • Main PCB

Related codes

Full explanation for H.01 – .00
H.01 – .05

Your boiler has detected that water is not moving through the system quickly enough or is getting too hot too fast, usually due to a blockage or a pump issue.

High DIY-safe £120-350

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulation Pump
  • Flow/Return Temperature Sensors (NTC)
  • Main Heat Exchanger (if blocked)
Full explanation for H.01 – .05
H.01 – .08

Your boiler is overheating or water isn't moving through the pipes quickly enough, causing the system to shut down to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge.
  2. Ensure all radiator valves and the main isolation valves under the boiler are fully open.
  3. Bleed your radiators to remove any trapped air pockets.

Parts commonly replaced

  • Circulation Pump
  • NTC Temperature Sensor
  • Main Heat Exchanger (De-scaling)

Related codes

Full explanation for H.01 – .08
H.01 – .14

Your boiler has detected that it is getting too hot too quickly, usually because water isn't moving through the system properly.

High DIY-safe £100-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge
  2. Ensure all radiator valves are fully open
  3. Check for any frozen pipes if the weather is extremely cold
  4. Reset the boiler to see if the temporary fault clears

Parts commonly replaced

  • Central heating pump
  • Flow temperature sensor
  • Main heat exchanger (flushing)
Full explanation for H.01 – .14
H.01 – .18

Your boiler has stopped working because water is not flowing through the system properly or a sensor has detected an unusual temperature rise.

High DIY-safe £120-£350

What to check first

  1. Check if the system pressure gauge is between 1 and 1.5 bar
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Bleed your radiators to remove any trapped air pockets
  4. Reset the boiler by turning the selector switch to 'R' for five seconds

Parts commonly replaced

  • Circulation pump
  • Temperature NTC sensors
  • Main heat exchanger (de-scaling)

Related codes

Full explanation for H.01 – .18
H.01 – .21

Your boiler is heating up too quickly when you run your hot water, causing it to shut down to prevent overheating.

High DIY-safe £100-280

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the pressure gauge
  2. Ensure your main water stop tap and any valves under the boiler are fully open
  3. Switch the boiler off and back on again to see if the error clears

Parts commonly replaced

  • DHW Temperature Sensor (NTC)
  • Plate Heat Exchanger
  • Internal Pump
Full explanation for H.01 – .21
H.02 – .02

Your boiler has lost its configuration settings and doesn't know how to operate properly, usually following a parts replacement or a power glitch.

High Engineer only £100-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Configuration Plug

Related codes

Full explanation for H.02 – .02
Showing 121–129 of 129
H.02 – .03

Your boiler's control board is temporarily confused by its internal setup settings and needs a quick restart to synchronise correctly.

Low DIY-safe £0-120

What to check first

  1. Locate the selector switch on the front of the boiler
  2. Turn the switch to the 'R' (Reset) position for at least 5 seconds
  3. Return the switch to its original position and wait for the boiler to restart

Parts commonly replaced

  • None (Software reset usually resolves)
  • Printed Circuit Board (PCB) if fault persists

Related codes

Full explanation for H.02 – .03
H.02 – .04

Your boiler's computer system is temporarily confused by its internal settings and cannot operate correctly.

High DIY-safe £100-350

What to check first

  1. Locate the reset button or selector switch on the front control panel.
  2. Press and hold the reset button for five seconds, then release.
  3. Wait for the boiler to complete its startup sequence to see if the error clears.

Parts commonly replaced

  • PCB (Main Control Board)
  • Parameter chip

Related codes

Full explanation for H.02 – .04
H.02 – .06

Your boiler has detected that the water pressure is too low or unstable, which usually means the system needs topping up or there is a small leak.

High DIY-safe £80-200

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. Locate the filling loop (usually two blue levers or a braided hose underneath the boiler) and slowly turn them to increase the pressure.
  3. Once the gauge reaches 1.5 bar, ensure both levers are fully closed and restart the boiler.

Parts commonly replaced

  • Filling loop valve
  • Auto air vent
  • Expansion vessel

Related codes

Full explanation for H.02 – .06
H.03 -.00

Your boiler has stopped working because its internal computer has lost contact with one of the temperature sensors.

High DIY-safe £90-180

What to check first

  1. Locate the power switch for the boiler and turn it off for 30 seconds.
  2. Turn the power back on and see if the error clears.
  3. If the code persists, check the boiler pressure gauge is between 1 and 1.5 bar.

Parts commonly replaced

  • Negative Temperature Coefficient (NTC) sensor
  • Flue sensor
  • Wiring harness

Related codes

Full explanation for H.03 -.00
H.03 – .01

Your boiler's internal computer is having trouble communicating with the main control system, causing the unit to stop working for safety.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness
  • Display interface board

Related codes

Full explanation for H.03 – .01
H.03 – .02

Your boiler was working but the flame has unexpectedly gone out, causing the system to stop for safety.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances (like a hob) are working to ensure gas is reaching the property
  2. Check if your prepayment gas meter has credit
  3. Press and hold the Reset button for five seconds

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for H.03 – .02
OFF OFF OFF

Your boiler is not receiving any electricity and its control screen has completely shut down.

High DIY-safe £0-250

What to check first

  1. Check if other appliances or lights in your home are working to rule out a power cut.
  2. Ensure the boiler's fused spur switch (usually a wall switch near the boiler) is turned on.
  3. Check your home's main consumer unit (fuse box) to see if a circuit breaker has tripped.

Parts commonly replaced

  • Internal PCB fuse
  • Printed Circuit Board (PCB)
  • External Fused Spur
Full explanation for OFF OFF OFF
ON OFF ON

Your boiler has tried to light several times but failed, so it has locked itself for safety.

High DIY-safe £100-280

What to check first

  1. Check that your gas pre-payment meter (if you have one) has credit
  2. Ensure other gas appliances, like a cooker hobs, are working to confirm gas supply
  3. Turn the selector knob to the 'Reset' position for five seconds and release

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing lead

Related codes

Full explanation for ON OFF ON
ON ON ON

Your boiler has detected an internal electrical communication problem and needs a professional inspection to restore power to its controls.

High Engineer only £120-250

Parts commonly replaced

  • Low voltage wiring harness
  • PCB (Main Control Board)

Related codes

Full explanation for ON ON ON