Boiler Fault Code E.00.21

Also written as E0021 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code E.00.21 appears across 1 boiler brand and 7 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has detected that the exhaust gases are getting too hot or the internal sensor that monitors them has developed a wiring fault, causing the system to shut down for safety. (As reported on Baxi 800)