BoilerFaultCodesThe 6 most common faults
01
EA High DIY-safe
Your boiler is failing to light the gas or isn't detecting that the flame has started, causing the system to shut down for safety.
What to try first
Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.If it is freezing outside, check that your external condensate pipe hasn't frozen and blocked.Press and hold the reset button for five seconds to see if the boiler restarts.
02
A6 High Engineer
Your boiler has detected an internal communication failure between its main electronic circuit boards, meaning it has temporarily stopped working for safety reasons.
Full guide for A603
A8 High Engineer
Your boiler has detected an internal electronic failure within its main control module, meaning it has safely shut down to prevent further issues.
Full guide for A804
b1 High Engineer
Your boiler's computer memory chip is loose or missing, meaning the system doesn't know which model it is and has stopped working for safety.
Full guide for b105
C1 High Engineer
Your boiler has stopped working because the internal fan, which clears exhaust gases, isn't spinning fast enough to operate safely.
Full guide for C106
d3 High Engineer
Your boiler has temporarily stopped working because a vital safety connection inside the unit has become loose or disconnected.
Full guide for d3on the Worcester Bosch Greenstar Highflow
Ignition ElectrodeFlame Sensor (Ionisation Probe)Gas ValvePCBWiring harnessPrinted Circuit Board (PCB)
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