13 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 |
|---|---|---|
| 41-406-04 | Greenstar FS 42CDi Regular ErP | 2008–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.
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Your boiler's internal computer is having trouble talking to its main control unit, which prevents it from operating correctly.
Your boiler's computer memory chip is loose or missing, meaning the system doesn't know which model it is and has stopped working for safety.
Your boiler's main control computer is experiencing an internal electronic fault and has stopped the system from operating to keep you safe.
Your boiler has detected that the internal fan responsible for safely venting exhaust gases is not working correctly, causing the system to stop heating to ensure your safety.
Your boiler has detected that waste water cannot drain away properly, so it has shut down to prevent internal water damage.
Your boiler's temperature sensor is faulty, meaning it cannot accurately read how hot the water is, so it has safely shut down to prevent overheating.
Your boiler has detected that it has become dangerously overheated and has safely shut itself down to prevent internal damage.
Your boiler has temporarily stopped working because it cannot detect a flame to safely heat your home.
Your boiler's internal computer has encountered a critical electronic software or hardware error that is preventing it from running safely.
Your boiler has detected an internal software error and has automatically shut down to prevent incorrect operation.
Your boiler has sensed a flame inside after it has tried to turn off, which is a safety lockout that requires professional attention.
Your boiler is still detecting a flame even though it has tried to shut off the gas, which is a safety conflict causing the system to lock out.
Your boiler has registered that the reset button has been pressed too many times within a short period and has temporarily locked itself for safety.
BoilerFaultCodes