BoilerFaultCodesThe 6 most common faults
01
A8 High Engineer
Your boiler has lost the ability for its internal parts to 'talk' to each other, so it has shut down to prevent errors.
Full guide for A802
AC High Engineer
Your boiler's electronic control system is failing to communicate with its internal components, causing the system to shut down for safety.
Full guide for AC03
b1 High Engineer
Your boiler's control panel has lost its connection to a crucial internal safety plug, meaning the system cannot communicate properly to function.
Full guide for b104
C1 High Engineer
Your boiler is struggling to push air through its internal system effectively, which stops it from firing up safely.
Full guide for C105
d3 High Engineer
Your boiler has lost the connection to its external thermostat or control unit, preventing it from knowing when to turn the heating on.
Full guide for d306
E2 High Engineer
Your boiler's temperature sensor is providing faulty information, causing the system to stop heating for safety reasons.
Full guide for E2on the Worcester Bosch ZB 7 27 HE
Printed Circuit Board (PCB)Communication wiring loomControl modulePCBWiring harnessKeyed plug
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