49 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
The model name and number are on the data badge under the boiler. Each generation has its own manual and fault list, so pick the exact one.
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
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Your boiler's main electronic control board has temporarily lost connection with the display screen, meaning the system cannot coordinate heating or hot water.
Your boiler has recorded a minor system event or the error memory needs clearing, which usually does not affect its operation.
Your boiler has lost the connection to the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety reasons.
Your boiler has detected a communication issue with the built-in hot water storage cylinder temperature sensor.
Your boiler has lost communication with the built-in hot water storage tank sensor, which means it may struggle to provide consistent hot water.
Your boiler has stopped working because one of its internal sensors has developed an electrical fault and can no longer measure the temperature of the water leaving the unit.
Your boiler has detected an electrical fault with the sensor that monitors water returning from your radiators, causing the system to shut down for safety.
Your boiler has detected an electrical wiring or sensor fault related to how it heats your hot water cylinder.
Your boiler has detected an electrical fault with the sensor that monitors your hot water tank temperature, preventing it from heating your water correctly.
Your boiler has switched itself off because it has detected that the water inside is getting too hot.
Your boiler has stopped working because the water pressure is too low for it to run safely.
Your boiler is struggling to move water around the system properly, causing it to overheat and shut down for safety.
Your boiler is heating up too quickly because water isn't flowing through the system properly, causing it to shut down to prevent overheating.
Your boiler has detected an electrical connection issue or a sudden temperature rise, causing it to shut down for safety.
Your boiler is unable to open the internal valve that supplies gas to the burner, meaning it cannot ignite to provide heat or hot water.
Your boiler's safety system is detecting a flame when there shouldn't be one, or the sensor that monitors the flame has developed a fault.
Your boiler is not receiving gas or is unable to light a flame to provide heating and hot water.
Your boiler has lost its flame while running and is now unable to relight itself to provide heat or hot water.
Your boiler has detected an issue with the air fan, meaning it cannot safely circulate the air and gas needed to stay lit.
Your boiler has detected a blockage or issue with the pipe that brings in fresh air and removes exhaust gases.
Your boiler has lost its memory of which specific model it is, meaning it cannot operate safely without professional reconfiguration.
Your boiler has lost communication with its external controls, such as your thermostat, often due to an electrical wiring error or interference.
Your boiler has lost communication with a sensor that monitors the flow of air and gas, so it has safely shut down to prevent incorrect operation.
Your boiler has temporarily stopped working because it cannot properly measure the flow of air and gas needed for safe operation.
Your boiler has temporarily stopped working because it is not detecting enough gas supply to safely light the flame.
Your boiler has detected an issue with how it regulates the flow of gas and air inside, so it has temporarily locked itself out for safety.
Your boiler has detected an issue while trying to operate safely in its high-performance mode, so it has temporarily locked itself out to prevent any potential risks.
Your boiler has detected an electrical communication problem with the gas valve, which is preventing it from lighting safely.
Your boiler has detected a potential safety issue where the gas valve is not closing properly or the control board is misreading the gas flow.
Your boiler's internal computer brain has developed a memory error and can no longer process the information it needs to run safely.
Your boiler has detected an electrical fault or a sensor failure and has shut down as a safety precaution.
Your boiler’s internal computer has overheated, causing it to shut down to prevent permanent damage.
Your boiler's internal computer has detected an electrical fault with the flame sensor and has shut down for safety.
Your boiler has temporarily shut down because the flame burning inside it is flickering or becoming unstable.
Your boiler's internal computer parts have lost their identity settings or been replaced incorrectly, meaning the system can no longer communicate with itself to operate.
Your boiler has detected that a temperature sensor is stuck or faulty, meaning it cannot accurately monitor the water temperature to operate safely.
Your boiler’s temperature sensors are giving conflicting readings, causing the system to shut down to prevent overheating.
Your boiler has lost the ability to monitor its own water pressure due to a sensor or wiring fault, causing the system to shut down for safety.
Your boiler is struggling to read its internal water pressure correctly because of a technical glitch in the sensor or its wiring.
Your boiler is unable to detect a change in water pressure when the pump starts up, preventing it from firing up for heating or hot water.
Your boiler has shut down because a safety sensor has detected that the main heating component is getting too hot or has a broken internal fuse.
Your boiler has shut down because it is unable to drain away the waste water it produces, or a safety sensor on the flue system has been triggered.
Your boiler is confused because it is trying to communicate with an external control box that isn't compatible with its current setup.
Your boiler has detected that the water inside is not heating up or moving properly when the burner tries to start.
Your boiler has detected an unusual temperature difference between its internal pipes, usually because the monitoring sensors are misread, incorrectly attached, or recently swapped during maintenance.
Your boiler's temperature sensors have been attached to the wrong pipes by mistake, so the system cannot properly measure how water is flowing through it.
Your boiler has registered a secondary system error because another underlying fault occurred first.
Your boiler has detected a mismatch in its internal settings regarding the type of gas it is using, so it has safely shut down to prevent any risk.
Your boiler has detected an issue with the quality of combustion and the gas mixture, so it has locked itself out for safety.
Not faults — these are normal operating states. Tap to expand.
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BoilerFaultCodes