Baxi 800 System 2 Fault codes & diagnostics

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

System Natural Gas 2023-present
Choose your boiler type

The 800 range comes in 3 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

818 System 2 1 codes · 1 GC number 830 System 2 1 codes · 1 GC number 824 System 2 1 codes · 1 GC number
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Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 57 High 9 Medium 7 Low
Just want the most common Baxi 800 System 2 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 75 documented codes

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254

Your boiler has lost communication with its wall thermostat or controller, meaning it cannot properly receive heating instructions.

Medium DIY-safe £90-200 Shared across the 800 System 2 range

What to check first

  1. Check if the wireless room thermostat or control unit needs its batteries replaced.
  2. Power cycle the boiler and the control unit to re-establish their wireless or wired connection.
  3. Ensure the control unit is within range and not experiencing interference from other household electronics.

Parts commonly replaced

  • Wireless Room Thermostat
  • PCB
  • Receiver Unit

Related codes

Full explanation for 254
A.00.34

Your boiler is looking for an outdoor weather sensor that it can no longer communicate with, which might cause your heating to behave unpredictably.

Low DIY-safe £90-160 Shared across the 800 System 2 range

What to check first

  1. Check if any external wires on the outside of your house leading to a small plastic box have been damaged or cut.
  2. Try resetting the boiler by pressing the 'R' or Reset button for five seconds.
  3. If you recently had building work near the boiler or outside walls, check if the sensor was accidentally unplugged.

Parts commonly replaced

  • Outdoor temperature sensor
  • Wiring harness
  • Sensor cable

Related codes

Full explanation for A.00.34
A.02.06

Your boiler has detected that there is not enough water in the central heating system to operate safely.

High DIY-safe £0-120 Shared across the 800 System 2 range

What to check first

  1. Locate the filling loop (the silver braided hose) underneath the boiler.
  2. Slowly open the blue or black taps on the filling loop until you hear water flowing.
  3. Monitor the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for A.02.06
A.02.18

Your boiler's computer is struggling to communicate with its internal control system, causing the unit to stop working to prevent an unsafe condition.

High Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB
  • Wiring harness

Related codes

Full explanation for A.02.18
A.02.36

Your boiler has lost communication with one of its internal control components, meaning it cannot operate safely and has shut itself down.

High Engineer only £120-£350 Shared across the 800 System 2 range

Parts commonly replaced

  • Wiring harness
  • Printed Circuit Board (PCB)
  • Low pressure switch

Related codes

Full explanation for A.02.36
A.02.37

Your boiler has lost communication with a sensor or peripheral device, which is causing it to stop operating to ensure safety.

High Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Wiring harness
  • PCB
  • Sensor

Related codes

Full explanation for A.02.37
A.02.45

Your boiler is having a communication problem where the internal parts are temporarily unable to 'talk' to each other, causing the system to stop working.

High Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for A.02.45
A.02.46

Your boiler is struggling to manage the demand between heating your radiators and heating your water, causing the system to pause operations.

Medium Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Diverter valve
  • PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for A.02.46
A.02.48

Your boiler's internal computer has lost its specific settings, meaning it has forgotten how to correctly control your heating system.

High Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for A.02.48
A.02.49

Your boiler's internal computer is failing to start up properly, meaning it cannot communicate with its internal parts to turn on.

High Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for A.02.49
Showing 11–20 of 75
A.02.54

Your boiler is having trouble communicating with your thermostat, which is preventing it from receiving the signal to turn on your heating.

High Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • OpenTherm wiring harness

Related codes

Full explanation for A.02.54
A.02.55

Your boiler's internal computer has lost its identification data or is not communicating correctly with its main control board.

High Engineer only £120-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal Data Key

Related codes

Full explanation for A.02.55
A.02.76

Your boiler's computer has reached a limit where it can no longer store or accept new configuration changes, but it is still operating normally.

Low Engineer only Shared across the 800 System 2 range

Parts commonly replaced

  • PCB (Printed Circuit Board)

Related codes

Full explanation for A.02.76
E.00 .04

Your boiler has lost its connection to the sensor that monitors water returning from your radiators, causing the system to stop working as a safety precaution.

High Engineer only £90-160 Shared across the 800 System 2 range

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness
  • Control PCB

Related codes

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

Your boiler has detected a fault with the sensor that measures the temperature of the water returning to the unit, causing it to stop running as a safety precaution.

High Engineer only £90-160 Shared across the 800 System 2 range

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

Full explanation for E.00 .05
E.00 .06

Your boiler has lost connection with a temperature sensor that monitors water returning to the unit, causing the system to stop working as a safety precaution.

High Engineer only £90-180 Shared across the 800 System 2 range

Parts commonly replaced

  • Return NTC thermistor sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for E.00 .06
E.00 .07

Your boiler has sensed an unusual temperature gap between the water leaving and returning to the unit, suggesting the water isn't flowing through your heating system correctly.

High DIY-safe £100-£350 Shared across the 800 System 2 range

What to check first

  1. Ensure all radiator valves (TRVs) are fully open
  2. Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge
  3. Gently bleed your radiators to remove trapped air

Parts commonly replaced

  • Central heating pump
  • Return temperature sensor (NTC)
  • Main heat exchanger (flushing/cleaning)

Related codes

Full explanation for E.00 .07
E.00 .16

Your boiler is unable to read the temperature of your hot water cylinder, which means you may not have reliable hot water.

High Engineer only £90-180 Shared across the 800 System 2 range

Parts commonly replaced

  • DHW Tank Temperature Sensor (NTC)
  • Connection wiring/harness

Related codes

Full explanation for E.00 .16
E.00 .17

Your boiler has detected a fault with the sensor that monitors your hot water temperature, meaning it cannot safely heat your water to the correct level.

Medium Engineer only £90-160 Shared across the 800 System 2 range

Parts commonly replaced

  • DHW temperature sensor (NTC thermistor)
  • Sensor wiring harness

Related codes

Full explanation for E.00 .17
E.00.20

Your boiler has detected an issue with the sensor that monitors the safety of exhaust gases, causing the system to shut down for protection.

High Engineer only £120-£200 Shared across the 800 System 2 range

Parts commonly replaced

  • Flue gas temperature sensor (NTC)
  • Wiring harness
  • Main circuit board (PCB)

Related codes

Full explanation for E.00.20
Showing 21–30 of 75
E.00.21

Your boiler has detected that the exhaust gases are getting too hot or the internal sensor that monitors them has developed a wiring fault, causing the system to shut down for safety.

High Engineer only £120-£220 Shared across the 800 System 2 range

Parts commonly replaced

  • Flue Gas Temperature Sensor
  • Sensor Wiring Harness
  • Main Heat Exchanger

Related codes

Full explanation for E.00.21
E.01.04

Your boiler is repeatedly struggling to keep the flame lit while it is running, causing it to shut down for safety after multiple failures.

High DIY-safe £120-350 Shared across the 800 System 2 range

What to check first

  1. Check that your gas pre-payment meter has credit
  2. Ensure other gas appliances in your home, like a hob, are working correctly
  3. Reset the boiler by pressing the 'R' or 'Reset' button

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for E.01.04
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 there is a circulation problem or a sensor fault.

High DIY-safe £90-220 Shared across the 800 System 2 range

What to check first

  1. Ensure all radiator valves (TRVs) are fully open
  2. Check that the boiler pressure is between 1.0 and 1.5 bar
  3. Press the Reset button for five seconds

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Flow temperature sensor
  • Central heating pump

Related codes

Full explanation for E.01.12
E.01.17

Your boiler has detected that water is not moving through the system properly, which has caused it to shut down to prevent overheating.

High DIY-safe £100-£350 Shared across the 800 System 2 range

What to check first

  1. Check the pressure gauge and top it up to 1.5 bar using the filling loop if it is too low
  2. Ensure all radiator valves are fully open
  3. Check for any visible leaks around your pipework
  4. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for E.01.17
E.01.20

Your boiler has sensed that the exhaust gases are getting too hot and has shut itself down as a safety precaution.

High Engineer only £150-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Main Heat Exchanger
  • Heat Exchanger NTC Sensor
  • Flue Thermostat

Related codes

Full explanation for E.01.20
E.02.13

Your boiler has stopped because an external control, such as a weather compensation sensor or a smart thermostat, is sending a signal that it should not be running.

Medium Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • External sensor
  • Wiring loom
  • Main control PCB

Related codes

Full explanation for E.02.13
E.02.15

Your boiler's main internal computer has lost contact with one of the secondary circuit boards, meaning the system cannot talk to itself properly to coordinate the heating.

High Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Secondary/External PCB

Related codes

Full explanation for E.02.15
E.02.17

Your boiler has stopped working because the internal control board and the gas valve have lost the ability to talk to each other.

High Engineer only £180-450 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB
  • Gas Valve
  • Wiring Harness

Related codes

Full explanation for E.02.17
E.02.35

Your boiler has detected that a vital safety component has become unplugged or has failed, causing it to shut down to prevent any risk of harm.

Emergency Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • Wiring harness
  • Safety sensor
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.02.35
E.02.47

Your boiler has lost its connection to an external control, such as your room thermostat or smart hub, meaning it cannot receive instructions on when to turn the heating on.

Medium Engineer only £120-220 Shared across the 800 System 2 range

Parts commonly replaced

  • External controls
  • Wiring harness
  • PCB

Related codes

Full explanation for E.02.47
Showing 31–40 of 75
E.04.00

Your boiler's internal computer has detected a configuration error in its safety settings, preventing it from operating to ensure your protection.

High Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.04.00
E.04.01

Your boiler has detected a fault with the internal sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely regulate heat.

High Engineer only £90-160 Shared across the 800 System 2 range

Parts commonly replaced

  • Flow Temperature Sensor (NTC Thermistor)
  • Wiring Harness

Related codes

Full explanation for E.04.01
E.04.02

Your boiler has lost communication with the internal thermometer that monitors water leaving the unit, causing the system to shut down for safety.

High Engineer only £100-180 Shared across the 800 System 2 range

Parts commonly replaced

  • Flow Temperature Sensor (NTC thermistor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for E.04.02
E.04.03

Your boiler has shut down because it thinks it is getting too hot or it has detected a technical fault with its internal temperature sensor.

High DIY-safe £90-180 Shared across the 800 System 2 range

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1 and 1.5 bar on the gauge
  3. Turn the boiler off and back on again to attempt a reset

Parts commonly replaced

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

Related codes

Full explanation for E.04.03
E.04.07

Your boiler has detected that the water temperature readings inside the unit do not match, causing it to shut down to prevent damage.

High Engineer only £150-250 Shared across the 800 System 2 range

Parts commonly replaced

  • NTC Temperature Sensors
  • Wiring harness
  • PCB

Related codes

Full explanation for E.04.07
E.04.08

Your boiler has shut down because the water inside has become far too hot, likely due to a circulation problem.

High DIY-safe £120-£350 Shared across the 800 System 2 range

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure there is enough water pressure in the system (usually between 1.0 and 1.5 bar on the gauge)
  3. Try resetting the boiler by turning the selector knob to 'R' for five seconds

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger (Cleaning/Flushing)

Related codes

Full explanation for E.04.08
E.04.09

Your boiler has detected an inconsistent temperature reading within its exhaust system, which has caused it to stop running to ensure safe operation.

High Engineer only £150-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Flue gas temperature sensors
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.04.09
E.04.10

Your boiler has tried and failed to light its burner four times, meaning it cannot provide heating or hot water until the issue is resolved.

High DIY-safe £120-350 Shared across the 800 System 2 range

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob
  2. Ensure your gas meter has credit and the handle is in the 'on' position
  3. Press the 'Reset' button on the boiler control panel for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Electrode
  • Condensate Trap/Pipe

Related codes

Full explanation for E.04.10
E.04.11

Your boiler has detected a safety check failure during the startup process, preventing the gas valve from opening correctly.

High Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Gas Valve
  • Electrode pack
  • PCB

Related codes

Full explanation for E.04.11
E.04.12

Your boiler is detecting a flame when there shouldn't be one, so it has shut down as a safety precaution.

High Engineer only £120-280 Shared across the 800 System 2 range

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.04.12
Showing 41–50 of 75
E.04.13

Your boiler's fan, which clears exhaust gases, is struggling to spin at the correct speed or is being physically obstructed.

High Engineer only £250-£450 Shared across the 800 System 2 range

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.04.13
E.04.14

Your boiler is struggling to control its internal temperature properly, meaning it has paused its heating cycle to prevent potential overheating.

Medium Engineer only £120-250 Shared across the 800 System 2 range

Parts commonly replaced

  • NTC Temperature Sensor
  • Printed Circuit Board (PCB)
  • Pump

Related codes

Full explanation for E.04.14
E.04.15

Your boiler has stopped working because the pipe that carries exhaust fumes outside is likely obstructed or blocked by debris.

High DIY-safe £80-180 Shared across the 800 System 2 range

What to check first

  1. Check the external flue pipe outside your home for any visible obstructions like bird nests, leaves, or snow
  2. Ensure there are no items leaning against or covering the flue terminal
  3. Gently clear any external debris and restart the boiler using the reset button

Parts commonly replaced

  • Flue terminal
  • Fan assembly
  • Air pressure switch

Related codes

Full explanation for E.04.15
E.04.17

Your boiler has detected an internal electrical problem with the component that controls the gas supply, meaning it cannot safely light itself to provide heat or hot water.

High Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

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

Related codes

Full explanation for E.04.17
E.04.18

Your boiler has detected that a temperature sensor is either disconnected or reading an unusually cold temperature, preventing it from heating your home safely.

High Engineer only £90-180 Shared across the 800 System 2 range

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for E.04.18
E.04.19

Your boiler is having trouble communicating with an internal sensor that monitors the flow of water, causing it to stop working to ensure your safety.

High Engineer only £150-250 Shared across the 800 System 2 range

Parts commonly replaced

  • Flow sensor
  • Wiring harness
  • PCB

Related codes

Full explanation for E.04.19
E.04.20

Your boiler is struggling to accurately measure the movement of water inside the system, which is causing it to shut down to prevent potential overheating.

High Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Flow sensor
  • Wiring harness
  • Printed Circuit Board

Related codes

Full explanation for E.04.20
E.04.21

Your boiler has detected an inconsistent temperature reading between its internal sensors, which causes it to shut down to prevent potential overheating.

High Engineer only £150-250 Shared across the 800 System 2 range

Parts commonly replaced

  • NTC Temperature sensors
  • Wiring harness
  • Printed Circuit Board

Related codes

Full explanation for E.04.21
E.04.23

Your boiler has lost connection with its gas control unit, meaning it cannot safely release fuel to start the flame.

High Engineer only £220-450 Shared across the 800 System 2 range

Parts commonly replaced

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

Related codes

Full explanation for E.04.23
E.04.24

Your boiler is struggling to identify the type of gas being supplied, causing it to shut down for safety.

High DIY-safe £100-250 Shared across the 800 System 2 range

What to check first

  1. Check that your gas meter is turned on and you have credit if using a prepay meter
  2. Check if other gas appliances in your home, like a hob, are working correctly
  3. Reset the boiler by pressing the reset button for five seconds

Parts commonly replaced

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

Related codes

Full explanation for E.04.24
Showing 51–60 of 75
E.04.54

Your boiler has detected an internal electrical problem preventing it from safely controlling the gas supply, so it has shut itself down to keep you safe.

Emergency Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

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

Related codes

Full explanation for E.04.54
H.00.42

Your boiler has stopped working because it cannot correctly detect the water pressure, likely due to a faulty internal sensor or a broken connection.

High Engineer only £140-£220 Shared across the 800 System 2 range

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for H.00.42
H.01 .00

Your boiler has stopped working because the internal control board and the gas supply valve are temporarily failing to talk to each other.

High Engineer only £150-£450 Shared across the 800 System 2 range

Parts commonly replaced

  • Gas valve
  • Boiler PCB (Printed Circuit Board)
  • Wiring harness

Related codes

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

Your boiler has temporarily stopped because the water leaving the boiler is getting much hotter than the water returning from your radiators.

High Engineer only £150-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Circulating pump
  • Flow/return thermistors
  • Automatic bypass valve

Related codes

Full explanation for H.01 .05
H.01 .08

Your boiler is temporarily pausing because the water inside is heating up much faster than normal.

Low DIY-safe £80-200 Shared across the 800 System 2 range

What to check first

  1. Check that your radiator valves are fully open to allow water to flow freely.
  2. Make sure your heating system pressure is set correctly between 1 and 1.5 bar.
  3. Restart the boiler once it has had 10 minutes to cool down.

Parts commonly replaced

  • Automatic bypass valve
  • Circulating pump
  • NTC temperature sensor

Related codes

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

Your boiler has temporarily shut down because the water inside it has become too hot, usually caused by a blockage in water flow or a circulation issue.

Medium DIY-safe £100-£250 Shared across the 800 System 2 range

What to check first

  1. Check that all your radiators are switched on and their valves are fully open to allow water to circulate freely.
  2. Verify that your system pressure is correct, typically between 1 and 1.5 bar.
  3. Restart or reset the boiler using its control panel once the system has cooled down.

Parts commonly replaced

  • Circulating pump
  • Automatic bypass valve
  • NTC temperature sensor

Related codes

Full explanation for H.01 .14
H.01 .18

Your boiler has detected that water is not moving through the system properly, often because the water pressure is too low.

High DIY-safe £0-120 Shared across the 800 System 2 range

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 (the silver braided hose) underneath the boiler.
  3. Open the valves slowly to top up the water pressure until the gauge reaches 1.2 bar, then close them tightly.
  4. Restart the boiler to clear the fault.

Parts commonly replaced

  • N/A (Usually just a pressure top-up)
  • Filling loop valve
  • Expansion vessel

Related codes

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

Your boiler has temporarily paused because the water temperature inside is rising too quickly, and it will try to restart automatically in 10 minutes.

Low Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • NTC thermistor
  • Pump
  • Heat exchanger

Related codes

Full explanation for H.01 .21
H.02 .00

Your boiler is currently restarting itself or finishing a reset cycle and should return to normal operation shortly.

Low DIY-safe £0 Shared across the 800 System 2 range

What to check first

  1. Wait up to 5 minutes for the countdown or reset process to complete
  2. Ensure any 'Reset' buttons are not stuck in the pressed position
  3. If the code persists for more than 10 minutes, try turning the main power switch off and back on

Related codes

Full explanation for H.02 .00
H.02 .02

Your boiler is waiting for its control settings to be programmed, which usually happens during a new installation or if the main computer board has been replaced.

High Engineer only £90-150 Shared across the 800 System 2 range

Related codes

Full explanation for H.02 .02
Showing 61–70 of 75
H.02 .03

Your boiler’s internal computer has lost its specific identity settings and needs to be told exactly which model it is before it can operate.

High Engineer only £100-£250 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

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

Your boiler's main control board is having trouble reading its own saved internal settings, which is preventing the system from starting up.

High Engineer only £280-500 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Configuration Key

Related codes

Full explanation for H.02 .04
H.02 .05

Your boiler’s internal control board is confused because its software does not match the hardware installed, often following a repair or a power glitch.

High Engineer only £250-£450 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Parameters Key

Related codes

Full explanation for H.02 .05
H.02 .07

Your boiler has temporarily stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe Shared across the 800 System 2 range

What to check first

  1. Locate the filling loop or key underneath your boiler or near the pipework.
  2. Open the valves slowly to let water into the system until the pressure gauge on the front reaches between 1 and 1.5 bar.
  3. Close the valves tightly and press the reset button on your boiler if it does not restart automatically.

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for H.02 .07
H.02 .09

Your boiler is struggling to pump water properly because there is a blockage or restriction somewhere in the system.

High Engineer only £150-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Central heating pump
  • Plate heat exchanger
  • Magnetic system filter

Related codes

Full explanation for H.02 .09
H.02 .10

Your boiler has completely stopped working as a safety precaution and needs a qualified heating engineer to inspect it.

High Engineer only £150-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Gas valve

Related codes

Full explanation for H.02 .10
H.02 .12

Your boiler has stopped running because it has lost the signal from your external heating controls, such as your room thermostat or programmer.

High DIY-safe £0-120 Shared across the 800 System 2 range

What to check first

  1. Check if your room thermostat or programmer has power or needs new batteries.
  2. Ensure your smart home hub or Wi-Fi router is switched on and connected.
  3. Verify that the heating is actually turned 'on' at your wall controls.

Parts commonly replaced

  • Thermostat batteries
  • External room thermostat
  • Wiring harness

Related codes

Full explanation for H.02 .12
H.02 .38

Your boiler's water treatment or softening system isn't providing the required information about your water quality, which can lead to limescale buildup over time.

Low DIY-safe £60-120 Shared across the 800 System 2 range

What to check first

  1. Check if your external water softener is turned on and has salt
  2. Press the 'Reset' button on the front of the boiler
  3. Consult your water softener manual to ensure the communication cable is plugged into the boiler link

Parts commonly replaced

  • External water softener salt
  • Communication interface cable
  • Low voltage sensor

Related codes

Full explanation for H.02 .38
H.02 .70

Your boiler has detected an issue with its heat recovery system, which is preventing it from functioning normally and providing heating or hot water.

High Engineer only £150-300 Shared across the 800 System 2 range

Parts commonly replaced

  • Heat recovery sensor
  • Control PCB
  • Wiring harness

Related codes

Full explanation for H.02 .70
H.03 .00

Your boiler's main control board is unable to communicate with its internal safety chip, meaning the boiler has disabled itself for protection.

High Engineer only £180-350 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Safety Key
  • Wiring Harness

Related codes

Full explanation for H.03 .00
Showing 71–75 of 75
H.03 .01

Your boiler's electronic brain is unable to communicate with its internal control system, meaning it has switched itself off to prevent errors.

High Engineer only £250-450 Shared across the 800 System 2 range

Parts commonly replaced

  • Main PCB
  • Wiring harness
Full explanation for H.03 .01
H.03 .02

Your boiler's burner has unexpectedly gone out while running, causing the system to restart or stop to ensure safety.

Medium DIY-safe £80-250 Shared across the 800 System 2 range

What to check first

  1. Check that your gas pre-pay meter has credit (if applicable)
  2. Check that other gas appliances in your home are working correctly
  3. Press the Reset button for five seconds
  4. Ensure the external condensate pipe is not frozen if it is currently cold weather

Parts commonly replaced

  • Ionisation probe
  • Ignition electrode
  • Gas valve

Related codes

Full explanation for H.03 .02
H.03 .05

Your boiler is not receiving enough electrical power from your home's mains supply to operate safely.

High DIY-safe £100-£350 Shared across the 800 System 2 range

What to check first

  1. Check if there is a partial power cut or 'brownout' in your local area
  2. Ensure the boiler's fused spur switch is fully engaged and not damaged
  3. Switch the boiler off at the main spur, wait 30 seconds, and switch it back on to reset the electronics

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Voltage Stabiliser
  • Main Power Cable

Related codes

Full explanation for H.03 .05
H.03 .17

Your boiler is simply performing a routine internal safety test and should return to normal operation shortly.

Low DIY-safe £0 Shared across the 800 System 2 range

What to check first

  1. Wait for the boiler to complete the test (usually takes a few minutes)
  2. Check if the display returns to the normal temperature reading
  3. No action is required unless the code remains on the screen for an extended period

Related codes

Full explanation for H.03 .17
H.03 .54

Your boiler has temporarily shut down because the electrical voltage supplying it has dropped too low.

Medium Engineer only £90-200 Shared across the 800 System 2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Power Supply Unit

Related codes

Full explanation for H.03 .54
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