BoilerFaultCodes 254
Fault in Room Unit/Control Unit
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
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Severity at a glance
Each cell = one fault code. Hover to identify. 2 Emergency
37 High
4 Medium
2 Low
30 Unrated
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
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.
Your boiler is looking for an outdoor weather sensor that it can no longer communicate with, which might cause your heating to behave unpredictably.
What to check first
- Check if any external wires on the outside of your house leading to a small plastic box have been damaged or cut.
- Try resetting the boiler by pressing the 'R' or Reset button for five seconds.
- 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
A.02.06 Your boiler has detected that there is not enough water in the central heating system to operate safely.
Your boiler has detected that there is not enough water in the central heating system to operate safely.
What to check first
- Locate the filling loop (the silver braided hose) underneath the boiler.
- Slowly open the blue or black taps on the filling loop until you hear water flowing.
- 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
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.
Your boiler's computer is struggling to communicate with its internal control system, causing the unit to stop working to prevent an unsafe condition.
Parts commonly replaced
- Main PCB
- Wiring harness
Related codes
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.
Your boiler has lost communication with one of its internal control components, meaning it cannot operate safely and has shut itself down.
Parts commonly replaced
- Wiring harness
- Printed Circuit Board (PCB)
- Low pressure switch
Related codes
A.02.37 Your boiler has lost communication with a sensor or peripheral device, which is causing it to stop operating to ensure safety.
Your boiler has lost communication with a sensor or peripheral device, which is causing it to stop operating to ensure safety.
Parts commonly replaced
- Wiring harness
- PCB
- Sensor
Related codes
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.
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.
Parts commonly replaced
- Wiring harness
- Printed Circuit Board (PCB)
Related codes
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.
Your boiler is struggling to manage the demand between heating your radiators and heating your water, causing the system to pause operations.
Parts commonly replaced
- Diverter valve
- PCB (Printed Circuit Board)
- Wiring harness
Related codes
A.02.48 Your boiler's internal computer has lost its specific settings, meaning it has forgotten how to correctly control your heating system.
Your boiler's internal computer has lost its specific settings, meaning it has forgotten how to correctly control your heating system.
Parts commonly replaced
- Printed Circuit Board (PCB)
Related codes
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.
Your boiler's internal computer is failing to start up properly, meaning it cannot communicate with its internal parts to turn on.
Parts commonly replaced
- Printed Circuit Board (PCB)
- Wiring harness
Related codes
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.
Your boiler is having trouble communicating with your thermostat, which is preventing it from receiving the signal to turn on your heating.
Parts commonly replaced
- Printed Circuit Board (PCB)
- OpenTherm wiring harness
Related codes
A.02.55 Your boiler's internal computer has lost its identification data or is not communicating correctly with its main control board.
Your boiler's internal computer has lost its identification data or is not communicating correctly with its main control board.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
- Internal Data Key
Related codes
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.
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.
Parts commonly replaced
- PCB (Printed Circuit Board)
Related codes
E.00 .04
Return temperature sensor disconnected
E.00 .05
Return temperature sensor short circuited
E.00 .06
Return sensor expected but not detected
E.00 .07
The difference in return temperature is too great
E.00 .16
DHW tank temperature sensor not connected
E.00 .17
DHW tank temperature sensor short-circuited
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.
Your boiler has detected an issue with the sensor that monitors the safety of exhaust gases, causing the system to shut down for protection.
Parts commonly replaced
- Flue gas temperature sensor (NTC)
- Wiring harness
- Main circuit board (PCB)
Related codes
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.
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.
Parts commonly replaced
- Flue Gas Temperature Sensor
- Sensor Wiring Harness
- Main Heat Exchanger
Related codes
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.
Your boiler is repeatedly struggling to keep the flame lit while it is running, causing it to shut down for safety after multiple failures.
What to check first
- Check that your gas pre-payment meter has credit
- Ensure other gas appliances in your home, like a hob, are working correctly
- Reset the boiler by pressing the 'R' or 'Reset' button
Parts commonly replaced
- Flame sensing electrode
- Gas valve
- Ignition lead
Related codes
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.
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.
What to check first
- Ensure all radiator valves (TRVs) are fully open
- Check that the boiler pressure is between 1.0 and 1.5 bar
- Press the Reset button for five seconds
Parts commonly replaced
- Return temperature sensor (NTC thermistor)
- Flow temperature sensor
- Central heating pump
Related codes
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.
Your boiler has detected that water is not moving through the system properly, which has caused it to shut down to prevent overheating.
What to check first
- Check the pressure gauge and top it up to 1.5 bar using the filling loop if it is too low
- Ensure all radiator valves are fully open
- Check for any visible leaks around your pipework
- Restart the boiler using the reset button
Parts commonly replaced
- Circulation Pump
- Water Pressure Sensor
- Main Heat Exchanger (if blocked)
Related codes
E.01.20 Your boiler has sensed that the exhaust gases are getting too hot and has shut itself down as a safety precaution.
Your boiler has sensed that the exhaust gases are getting too hot and has shut itself down as a safety precaution.
Parts commonly replaced
- Main Heat Exchanger
- Heat Exchanger NTC Sensor
- Flue Thermostat
Related codes
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.
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.
Parts commonly replaced
- External sensor
- Wiring loom
- Main control PCB
Related codes
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.
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.
Parts commonly replaced
- Main PCB
- Wiring harness
- Secondary/External PCB
Related codes
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.
Your boiler has stopped working because the internal control board and the gas valve have lost the ability to talk to each other.
Parts commonly replaced
- Main PCB
- Gas Valve
- Wiring Harness
Related codes
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.
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.
Parts commonly replaced
- Wiring harness
- Safety sensor
- Printed Circuit Board (PCB)
Related codes
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.
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.
Parts commonly replaced
- External controls
- Wiring harness
- PCB
Related codes
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.
Your boiler's internal computer has detected a configuration error in its safety settings, preventing it from operating to ensure your protection.
Parts commonly replaced
- Printed Circuit Board (PCB)
Related codes
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.
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.
Parts commonly replaced
- Flow Temperature Sensor (NTC Thermistor)
- Wiring Harness
Related codes
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.
Your boiler has lost communication with the internal thermometer that monitors water leaving the unit, causing the system to shut down for safety.
Parts commonly replaced
- Flow Temperature Sensor (NTC thermistor)
- Wiring harness
- Main PCB
Related codes
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.
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.
What to check first
- Check that all radiator valves are fully open
- Ensure the boiler pressure is between 1 and 1.5 bar on the gauge
- 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
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.
Your boiler has detected that the water temperature readings inside the unit do not match, causing it to shut down to prevent damage.
Parts commonly replaced
- NTC Temperature Sensors
- Wiring harness
- PCB
Related codes
E.04.08 Your boiler has shut down because the water inside has become far too hot, likely due to a circulation problem.
Your boiler has shut down because the water inside has become far too hot, likely due to a circulation problem.
What to check first
- Check that your radiator valves are fully open
- Ensure there is enough water pressure in the system (usually between 1.0 and 1.5 bar on the gauge)
- 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
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.
Your boiler has detected an inconsistent temperature reading within its exhaust system, which has caused it to stop running to ensure safe operation.
Parts commonly replaced
- Flue gas temperature sensors
- Wiring harness
- Printed Circuit Board (PCB)
Related codes
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.
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.
What to check first
- Check if your gas supply is active by testing another gas appliance like a hob
- Ensure your gas meter has credit and the handle is in the 'on' position
- 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
E.04.11 Your boiler has detected a safety check failure during the startup process, preventing the gas valve from opening correctly.
Your boiler has detected a safety check failure during the startup process, preventing the gas valve from opening correctly.
Parts commonly replaced
- Gas Valve
- Electrode pack
- PCB
Related codes
E.04.12 Your boiler is detecting a flame when there shouldn't be one, so it has shut down as a safety precaution.
Your boiler is detecting a flame when there shouldn't be one, so it has shut down as a safety precaution.
Parts commonly replaced
- Flame sensing electrode
- Ignition lead
- Printed Circuit Board (PCB)
Related codes
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.
Your boiler's fan, which clears exhaust gases, is struggling to spin at the correct speed or is being physically obstructed.
Parts commonly replaced
- Fan assembly
- Printed Circuit Board (PCB)
Related codes
E.04.14 Your boiler is struggling to control its internal temperature properly, meaning it has paused its heating cycle to prevent potential overheating.
Your boiler is struggling to control its internal temperature properly, meaning it has paused its heating cycle to prevent potential overheating.
Parts commonly replaced
- NTC Temperature Sensor
- Printed Circuit Board (PCB)
- Pump
Related codes
E.04.15 Your boiler has stopped working because the pipe that carries exhaust fumes outside is likely obstructed or blocked by debris.
Your boiler has stopped working because the pipe that carries exhaust fumes outside is likely obstructed or blocked by debris.
What to check first
- Check the external flue pipe outside your home for any visible obstructions like bird nests, leaves, or snow
- Ensure there are no items leaning against or covering the flue terminal
- 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
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.
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.
Parts commonly replaced
- Gas Valve
- Printed Circuit Board (PCB)
- Wiring Harness
Related codes
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.
Your boiler has detected that a temperature sensor is either disconnected or reading an unusually cold temperature, preventing it from heating your home safely.
Parts commonly replaced
- Flow NTC sensor
- Wiring harness
- Control PCB
Related codes
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.
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.
Parts commonly replaced
- Flow sensor
- Wiring harness
- PCB
Related codes
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.
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.
Parts commonly replaced
- Flow sensor
- Wiring harness
- Printed Circuit Board
Related codes
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.
Your boiler has detected an inconsistent temperature reading between its internal sensors, which causes it to shut down to prevent potential overheating.
Parts commonly replaced
- NTC Temperature sensors
- Wiring harness
- Printed Circuit Board
Related codes
E.04.23 Your boiler has lost connection with its gas control unit, meaning it cannot safely release fuel to start the flame.
Your boiler has lost connection with its gas control unit, meaning it cannot safely release fuel to start the flame.
Parts commonly replaced
- Gas valve
- Printed Circuit Board (PCB)
- Wiring harness
Related codes
E.04.24 Your boiler is struggling to identify the type of gas being supplied, causing it to shut down for safety.
Your boiler is struggling to identify the type of gas being supplied, causing it to shut down for safety.
What to check first
- Check that your gas meter is turned on and you have credit if using a prepay meter
- Check if other gas appliances in your home, like a hob, are working correctly
- Reset the boiler by pressing the reset button for five seconds
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- PCB (Printed Circuit Board)
Related codes
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.
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.
Parts commonly replaced
- Gas valve
- Printed Circuit Board (PCB)
- Wiring harness
Related codes
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.
Your boiler has stopped working because it cannot correctly detect the water pressure, likely due to a faulty internal sensor or a broken connection.
Parts commonly replaced
- Water pressure sensor
- Wiring harness
Related codes
H.01 .00
Temporary communication failure between gas valve and boiler PCB
H.01 .05
Maximum temperature difference value between flow and return reached
H.01 .08
Flow temperature increase in heating mode too quick. Temporary stoppage of 10 minutes
H.01 .14
Maximum flow temperature value reached
H.01 .18
No water circulation (temporary)
H.01 .21
Flow temperature increase in domestic water mode too quick. Temporary stoppage of 10 minutes
H.02 .00
Reset in progress
H.02 .02
Waiting for configuration settings to be entered (CN1,CN2)
Showing 61–70 of 75
H.02 .03
Configuration settings (CN1,CN2) not entered correctly
H.02 .04
PCB settings cannot be read
H.02 .05
Setting memory not compatible with the boiler PCB type
H.02 .07
Low pressure in heating circuit (permanently)
H.02 .09
Partial stoppage of the boiler
H.02 .10
Complete stoppage of the boiler
H.02 .12
Opening of the control unit input signal contact from the external device
H.02 .38
No water hardness
H.02 .70
External unit heat recovery test failed
H.03 .00
No identification data for boiler safety device
Showing 71–75 of 75
H.03 .01
Communication failure in comfort circuit (internal fault in boiler PCB)
H.03 .02
Temporary flame loss
H.03 .05
Power supply voltage too low
H.03 .17
Periodic safety check in progress
H.03 .54
Temporary flame loss. Shutdown due to the power supply voltage being too low
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