BoilerFaultCodesThe 6 most common faults
01
A1 High DIY-safe
Your boiler has stopped because there isn't enough water circulating inside the system to keep it running safely.
What to try first
Check the pressure gauge on the front of the boiler; if it is below 1 bar, your system needs topping up.Locate the filling loop (usually a silver braided hose under the boiler) and open the valves to let water in until the needle reaches 1.5 bar.Bleed your radiators to remove any trapped air, then re-check the pressure gauge and top up again if it has dropped.
02
C1 High Engineer
Your boiler has detected that the internal fan is moving too slowly to safely vent waste gases, so it has stopped working as a precaution.
Full guide for C103
C6 High Engineer
Your boiler has detected that the fan, which is essential for safely venting exhaust gases, is not spinning at the correct speed.
Full guide for C604
E2 High Engineer
Your boiler has lost the ability to read the temperature of the water used for your radiators, causing it to shut down for safety.
Full guide for E205
E9 High Engineer
Your boiler has detected that it is overheating and has automatically switched itself off to prevent damage or dangerous pressure levels.
Full guide for E906
EA High Engineer
Your boiler is failing to light the internal flame needed to heat your home's water.
Full guide for EAon the Worcester Bosch CDi RSF
Central heating pumpPressure relief valvePressure gaugeFan assemblyPrinted Circuit Board (PCB)NTC Sensor
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