Boiler Fault Code 40°C neon flashing

Also written as 40CNEONFLASHING — same fault.

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

Fault code 40°C neon flashing appears across 1 boiler brand and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is failing to detect the water flowing through it, which prevents it from firing up to provide your heating or hot water. (As reported on Potterton Performa 24i HE)