BoilerFaultCodesThe 6 most common faults
01
b1 High Engineer
Your boiler has lost its connection to an internal system component, meaning it cannot properly communicate with its own controls to function.
Full guide for b102
C1 High Engineer
Your boiler is struggling to push air through its internal system, which prevents it from safely igniting and providing heating or hot water.
Full guide for C103
d3 High Engineer
Your boiler is failing to start because it is not receiving a signal from your thermostat or internal control link to turn on the heating.
Full guide for d304
E2 High Engineer
Your boiler's internal thermometer is not sending a signal, so the system is shutting down to prevent overheating because it doesn't know how hot the water is.
Full guide for E205
E9 High Engineer
Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent any damage or safety issues.
Full guide for E906
EA High Engineer
Your boiler is trying to start but the flame is failing to light or stay on, so it has shut down as a safety precaution.
Full guide for EAon the Worcester Bosch Greenstar R29 40 HE
Keyed plug / Coding plugPCBFan assemblyPrinted Circuit Board (PCB)Flue sensorExternal controls/thermostat
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Worcester Bosch shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue