Baxi 800 Fault codes & diagnostics

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

Combi Natural Gas 25-36 kW 2023-present
GC Numbers 47-077-6347-077-6447-077-65
Download the Baxi 800 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
44 High 3 Medium 4 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-077-63 824 COMBI 2 2023–present
47-077-64 830 COMBI 2 2023–present
47-077-65 836 COMBI 2 2023–present
Just want the most common Baxi 800 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 51 documented codes

Tap any card for details

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 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 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.02.36

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 range

Parts commonly replaced

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

Related codes

Full explanation for A.02.36
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 range

Parts commonly replaced

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

Related codes

Full explanation for A.02.55
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 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 range

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

Full explanation for E.00.05
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 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 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 range

Parts commonly replaced

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

Related codes

Full explanation for E.00.20
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 range

Parts commonly replaced

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

Related codes

Full explanation for E.00.21
Showing 11–20 of 51
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 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 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 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 range

Parts commonly replaced

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

Related codes

Full explanation for E.01.20
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 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 range

Parts commonly replaced

  • Main PCB
  • Gas Valve
  • Wiring Harness

Related codes

Full explanation for E.02.17
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 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 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 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.04.08

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 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
Showing 21–30 of 51
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 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 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 range

Parts commonly replaced

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

Related codes

Full explanation for E.04.12
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 range

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E.04.13
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 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 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 range

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for E.04.18
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 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 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
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 range

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for H.00.42
Showing 31–40 of 51
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 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 detected that water is leaving the unit much hotter than when it returns, suggesting that the water isn't circulating through your radiators properly.

High DIY-safe £120-£350 Shared across the range

What to check first

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

Parts commonly replaced

  • Circulation Pump
  • Central Heating Filter (Cleaning)
  • Main Heat Exchanger (Flushing)

Related codes

Full explanation for H.01.05
H.01.08

Your boiler is heating up too quickly and has temporarily shut down to prevent overheating, usually because the water cannot circulate properly through your radiators.

High DIY-safe £80-250 Shared across the range

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge.
  2. Ensure at least two or three radiators have their valves fully open.
  3. Bleed your radiators to remove any trapped air that might be blocking water flow.

Parts commonly replaced

  • Central heating pump
  • Magnetic system filter (cleaning)
  • Main heat exchanger (flushing)

Related codes

Full explanation for H.01.08
H.01.14

Your boiler has shut down because it has detected that the water inside has become too hot, usually due to a lack of circulation.

High DIY-safe £120-350 Shared across the range

What to check first

  1. Ensure all radiator valves (TRVs) are turned to the highest setting
  2. Check that the boiler pressure is between 1 and 1.5 bar on the gauge
  3. Wait 30 minutes for the unit to cool down and press the Reset button

Parts commonly replaced

  • Central heating pump
  • Temperature thermistor (NTC sensor)
  • Main heat exchanger (flushing/cleaning)

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 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 is heating up too quickly when you turn on the hot water, causing it to shut down to prevent damage.

High Engineer only £120-£350 Shared across the range

Parts commonly replaced

  • Plate Heat Exchanger
  • Diverter Valve
  • Central Heating Pump

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 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 basically stuck in 'setup mode' because its main control board is waiting for essential configuration codes to be programmed in.

High Engineer only £90-180 Shared across the range

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for H.02.02
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 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 range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Configuration Key

Related codes

Full explanation for H.02.04
Showing 41–50 of 51
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 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 detected that the water pressure in your radiators is too low for the system to work safely, so it has turned itself off.

High DIY-safe £0-120 Shared across the range

What to check first

  1. Locate the flexible filling loop under the boiler with the two blue-handled taps.
  2. Slowly open both taps one at a time until you hear water flowing.
  3. Watch the pressure gauge on the front of the boiler until it reaches between 1.0 and 1.5 bar, then close both taps fully.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for H.02.07
H.02.09

Your boiler has detected low water flow and has automatically turned on its frost protection mode to prevent the system from seizing or freezing.

Medium DIY-safe £0-120 Shared across the range

What to check first

  1. Check that your boiler pressure is between 1.0 and 1.5 bar
  2. Ensure all radiator valves are fully open
  3. Check the condensate pipe outside for signs of freezing or blockage
  4. Restart the boiler using the reset button

Parts commonly replaced

  • System filter (cleaning)
  • Circulation pump
  • Pressure sensor

Related codes

Full explanation for H.02.09
H.02.10

Your boiler has shut down completely and its internal protection against freezing is no longer working, meaning your pipes could be at risk if the weather is very cold.

High Engineer only £150-350 Shared across the range

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal Wiring Harness

Related codes

Full explanation for H.02.10
H.02.12

Your boiler has been told to stop running by an external safety device or control, such as a thermostat or a condensation pump, which has detected an issue.

High DIY-safe £60-180 Shared across the range

What to check first

  1. Check if your thermostat or programmer is calling for heat and has working batteries.
  2. If you have a condensate pump, check if it is full or blocked, as it may have triggered a safety shut-off.
  3. Reset the boiler using the control knob to see if the error clears.

Parts commonly replaced

  • Condensate pump
  • External thermostat
  • Wiring harness

Related codes

Full explanation for H.02.12
H.02.38

Your boiler has detected that the water quality in the heating system is too soft or lacking the necessary minerals to function correctly.

Low Engineer only £90-180 Shared across the range

Parts commonly replaced

  • System Inhibitor Fluid
  • Water Hardness Sensor

Related codes

Full explanation for H.02.38
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 range

Parts commonly replaced

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

Related codes

Full explanation for H.03.00
H.03.01

Your boiler’s internal computer parts are struggling to talk to each other, which is preventing the system from operating correctly.

High Engineer only £250-£450 Shared across the range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness

Related codes

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 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 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
Showing 51–51 of 51
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 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
Boiler cover options

Avoid the next call-out bill

Monthly cover means no unexpected bills when your boiler breaks down.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.