42 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
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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 |
|---|---|---|
| 41-019-36 | SUSTAIN 12s | 2015–2016 |
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Your boiler's internal control screen cannot talk to the main computer brain, meaning it won't receive the signal to turn on.
Your boiler has lost connection with the sensor that monitors the temperature of the water leaving the unit, preventing it from heating up.
Your boiler has stopped working because it cannot communicate with the part that monitors the temperature of the water returning to the unit.
Your boiler has stopped working because it cannot correctly measure the temperature of the water leaving the unit due to an electrical connection issue.
Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water coming back into the unit, forcing it to stop for safety.
Your boiler has detected an electrical fault with the sensor that monitors your hot water temperature, meaning it cannot safely heat your water cylinder.
Your boiler has shut itself down because it has become too hot inside, likely due to a lack of water flow or a blockage.
Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Your boiler has shut down because the water is heating up too quickly or isn't circulating properly between the flow and return pipes.
Your boiler has shut down because it is heating up much faster than it should, which usually means water isn't moving through the system properly.
Your boiler has shut itself down because the exhaust gases have become too hot, which is a safety feature to prevent damage to the internal components.
Your boiler has sensed a flame even though it hasn't actually started the ignition process, causing it to shut down for safety.
Your boiler has tried to light several times but has been unable to establish a flame, meaning you will have no heating or hot water.
Your boiler was working but has unexpectedly turned itself off because the flame went out and it cannot manage to relight it.
Your boiler has stopped working because the fan that clears away waste gases is not spinning at the correct speed.
Your boiler has stopped working because it cannot confirm the fan is running correctly to safely clear away exhaust fumes.
Your boiler is experiencing a communication problem between its internal components due to an electrical voltage issue.
Your boiler has detected a communication problem with the gas valve, meaning it cannot safely light the burner to provide heat.
Your boiler has detected that the gas valve is not closing as quickly as it should, which is a safety feature to prevent unburnt gas from leaking into the system.
The boiler's internal computer memory has suffered a technical failure, meaning it can no longer process the instructions needed to run safely.
Your boiler has encountered a technical internal error where its electrical sensors and control board are failing to communicate correctly.
Your boiler's internal computer has reached too high a temperature and has shut down to prevent damage to the electronic components.
Your boiler's internal computer is receiving a confusing message about the flame, causing the system to shut down for safety.
Your boiler is struggling to keep a steady flame lit, causing it to shut down for safety.
Your boiler's internal computer brain doesn't recognise the identity of the unit, which often happens if a new control board has been installed but not correctly configured.
Your boiler has stopped working because it cannot accurately measure the temperature of the water leaving the unit.
Your boiler has sensed that the temperature sensors on the main pipes are disagreeing with each other, causing the system to shut down as a safety precaution.
Your boiler has stopped working because it cannot detect enough water pressure to run safely.
Your boiler has detected that the internal water pressure is too high or the sensor that measures it is reading incorrectly.
Your boiler has stopped working because it cannot detect the necessary change in water pressure required to safely start the heating process.
Your boiler has shut itself down because a safety sensor has detected that the main heating component is getting too hot.
Your boiler has detected an issue with getting rid of waste water or a blockage in the ventilation system, causing it to shut down for safety.
Your boiler has lost communication with the temperature sensor that monitors your hot water, meaning the system cannot safely determine the water temperature.
Your boiler has detected that the internal water temperature isn't rising as expected when the burner is on, which usually means there is air in the system or not enough water circulating.
Your boiler has noticed that its internal temperature sensors are giving conflicting readings and has shut down as a safety precaution.
Your boiler has detected that its temperature sensors are not reading correctly, likely because they have been fitted to the wrong pipes or are not making proper contact.
Your boiler has stopped running because it has detected a problem with the safety switch protecting your underfloor heating system.
Your boiler is unable to light the fuel to start the heating process because the part that creates the spark is not working correctly.
Your boiler has detected a problem with the internal mechanism that controls the gas supply, meaning it cannot light safely and has shut down.
Your boiler has stopped heating because the water is not being pumped around the system correctly.
Your boiler has lost the signal from the sensor that monitors your hot water tank, meaning it cannot accurately tell if your water is up to temperature.
Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Not faults — these are normal operating states. Tap to expand.
BoilerFaultCodes