BoilerFaultCodesThe 6 most common faults
01
A1 High DIY-safe
Your boiler has detected that there is not enough water circulating inside the system to run safely.
What to try first
Check the pressure gauge on the front of the boiler; if it is below 1 bar, locate the filling loopOpen the filling loop valves to top up the system pressure to between 1 and 1.5 barClose the filling loop valves tightly and bleed any trapped air from your radiators
02
A8 High Engineer
Your boiler's electronic controls have lost their connection, meaning the main brain cannot communicate with the internal components to start heating or hot water.
Full guide for A803
B1 High Engineer
Your boiler's computer is struggling to read its internal configuration settings, which prevents it from starting up correctly.
Full guide for B104
C6 High Engineer
Your boiler has switched itself off because the internal air circulation fan is not spinning fast enough to safely vent gases.
Full guide for C605
D3 High Engineer
Your boiler has stopped working because a safety link that tells the system it is safe to operate has become disconnected or faulty.
Full guide for D306
D5 High Engineer
Your boiler is struggling to get rid of the internal waste water it creates while running, causing it to shut down to prevent internal damage.
Full guide for D5on the Worcester Bosch CDi Highflow
System pressure sensorCentral heating pumpDiverter valvePrinted Circuit Board (PCB)BUS connection wiring harnessCode 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
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