20 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
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 |
|---|---|---|
| 41-592-73 | Ultra 18 System 18 | 2017–present |
| 41-592-73 | Ultra 18 System 18 | 2017–present |
Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.
Tap any card for details
Your boiler has lost the link to the valve that controls the gas supply, meaning it cannot ignite to provide you with heating or hot water.
Your boiler has detected an issue with the fuel supply system, meaning it cannot ignite safely to provide heating or hot water.
Your boiler has lost the ability to accurately measure the temperature of the water circulating through your radiators, so it has shut down to prevent overheating.
Your boiler has detected a communication error with its exhaust sensor, causing it to shut down for safety reasons.
Your boiler is struggling to accurately read the temperature of the water returning to it, which prevents the system from heating your home properly.
Your boiler has lost its internal settings, likely after a power cut or service, and requires a professional to recalibrate the electronic controls to ensure it burns gas safely and efficiently.
Your boiler has detected that the water is not circulating correctly before it attempts to ignite, so it has stopped working for safety reasons.
Your boiler has detected that it has become dangerously hot and has automatically shut itself down to prevent damage or fire risk.
Your boiler has detected that the water pressure inside your heating system has risen above the safe operating limit.
Your boiler has lost the necessary water pressure, so it has safely shut down to prevent damage.
Your boiler has detected that water is not moving effectively around your heating system, which forces the boiler to shut down to prevent overheating.
Your boiler has stopped working because it cannot light the internal flame or keep it burning reliably.
Your boiler has detected that the flue gases are becoming too hot, causing the system to shut down for safety reasons.
Your boiler has stopped working because it cannot light the internal flame, which is likely due to the gas supply being cut off or the ignition failing to spark.
Your boiler is failing to ignite because it detects that gas is not reaching the burner when it attempts to start.
Your boiler has stopped working because it is failing to receive a consistent supply of gas or is struggling to ignite properly.
Your boiler is struggling to compare the temperature of the water leaving and returning to the unit, so it has stopped working as a safety precaution.
Your boiler has stopped working because it cannot detect that the internal fan, which is essential for safely exhausting gases, is running correctly.
Your boiler is struggling to pump water around your heating system, which can cause it to overheat and shut down for safety.
Your boiler has detected a flame inside when it expects the internal fire to be completely off, which is a significant safety precaution.
Monthly cover means no unexpected bills when your boiler breaks down.
Affiliate links. We may earn a commission at no extra cost to you.
BoilerFaultCodes