BoilerFaultCodesThe 6 most common faults
01
A7 High Engineer
Your boiler is struggling to accurately measure the temperature of the water it is heating, which means you will likely have no hot water until it is fixed.
Full guide for A702
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 A803
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 b104
C1 High Engineer
Your boiler is struggling because the internal air circulation fan is not spinning fast enough to allow the boiler to fire up safely.
Full guide for C105
C4 High Engineer
Your boiler's fan is stuck running continuously, which wastes energy and prevents the boiler from starting correctly.
Full guide for C406
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 C6on the Worcester Bosch Greenstar R29 40 HE
Domestic Hot Water (DHW) NTC sensorControl Board (PCB)Wiring HarnessKeyed plug / Coding plugPCBFan assembly
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