BoilerFaultCodes Fault 03 / 21 Medium
Boiler Fault Code Fault 03 / 21
Also written as FAULT0321 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code Fault 03 / 21 appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler's built-in thermostat controls has developed an electrical fault and can no longer tell how warm your room is. (As reported on Worcester Bosch Greenstar CDi Compact)
Pick your boiler brand for the Fault 03 / 21 fix
Source
Information for the Fault 03 / 21 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026