BoilerFaultCodesThe 6 most common faults
01
A1 High DIY-safe
Your boiler has detected that the water pump is not moving water correctly, which often happens because there is air trapped in the system or the pressure is too low.
What to try first
Check your boiler's pressure gauge and ensure it reads between 1 and 1.5 barIf the pressure is below 1 bar, use the external filling loop to top up the systemBleed your radiators to remove any trapped air that might be preventing water circulation
02
C1 High Engineer
Your boiler is struggling to push air through the system to burn fuel safely, so it has stopped working to protect itself.
Full guide for C103
C4 High Engineer
Your boiler's internal fan is stuck running continuously, which prevents the system from starting up correctly or shutting down as it should.
Full guide for C404
C6 High Engineer
Your boiler has detected that the internal fan responsible for moving air and burnt gases is spinning faster than it should, causing the system to stop working for safety reasons.
Full guide for C605
d1 High Engineer
Your boiler has stopped working because it is unable to accurately measure the temperature of the water returning to it, so it has shut down as a precaution.
Full guide for d106
E2 High Engineer
Your boiler has lost the ability to accurately measure the temperature of the water flowing through it, so it has shut down as a precaution to prevent overheating.
Full guide for E2on the Worcester Bosch Ri 25 HE
Central heating pumpPressure relief valveExpansion vesselFan assemblyAir pressure switchWiring harness
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