F4 High

Boiler Fault Code F4

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code F4 appears across 9 boiler brands and 71 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 an issue with the sensor that measures the temperature of the water leaving the unit, preventing it from heating your home safely. (As reported on Ideal Logic Code Combi ESP1)

Pick your boiler brand for the F4 fix

Alpha F4 4 models Ferroli F4 5 models Glow-worm F4 7 models Heatline F4 10 models Ideal F4 17 models Keston F4 8 models Saunier Duval F4 9 models Vaillant F4 2 models Viessmann F4 9 models

Source

Information for the F4 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