BoilerFaultCodesThe 6 most common faults
01
H09 High Engineer
Your boiler has lost its electrical connection to the water pump, meaning it cannot circulate heat around your home.
Full guide for H0902
328 High Engineer
Your boiler's computer is struggling to get a steady power supply from your home's electricity, causing it to stop work to protect its internal parts.
Full guide for 32803
235 High Engineer
Your boiler's internal computer is reporting that a piece of hardware is using software that doesn't match the rest of the unit, preventing it from functioning.
Full guide for 23504
360 High Engineer
Your boiler's main electronic brain has lost its configuration or cannot read its own memory properly, so it has shut down for safety.
Full guide for 36005
363 High Engineer
Your boiler is struggling to detect that the flame is lit, so it has safely shut itself down to prevent wasting gas.
Full guide for 36306
H12 Medium Engineer
Your boiler is having trouble reading the temperature of your hot water cylinder, which means it may temporarily stop heating your water properly.
Full guide for H12on the Worcester Bosch Greenstar 21i System ErP
Pump cable harnessPrinted Circuit Board (PCB)Central heating pumpWiring harnessHCM (Heatronic Control Module)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
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