BoilerFaultCodesThe 6 most common faults
01
A8 High Engineer
Your boiler's internal electronic controls have lost their connection, preventing the heating and hot water from operating correctly.
Full guide for A802
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 b103
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 C104
C6 High Engineer
Your boiler has detected that the internal exhaust fan is spinning faster than it should, causing the system to stop working to keep your home safe.
Full guide for C605
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 d306
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 E2on the Worcester Bosch Greenstar R29 40 HE
Control Board (PCB)Wiring HarnessKeyed plug / Coding plugPCBFan assemblyPrinted Circuit Board (PCB)
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