How much does it cost to fix boiler error code E21?

Typical repair cost for the boiler E21 fault is £80-140 including labour.

E21 Emergency

Boiler Fault Code E21

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code E21 appears across 8 boiler brands and 35 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 its electrical wires are connected the wrong way round at the power source, preventing it from starting up safely. (As reported on Potterton Powermax HE)

Pick your boiler brand for the E21 fix

Ariston E21 4 models Biasi E21 6 models Ferroli E21 1 model Johnson Starley E21 7 models Potterton E21 2 models Ravenheat E21 9 models Saunier Duval E21 4 models Vokera E21 2 models

Source

Information for the E21 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