How much does it cost to fix boiler error code C42?

Typical repair cost for the boiler C42 fault is £120-250 including labour.

C42 High

Boiler Fault Code C42

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 7 July 2026

Fault code C42 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 has detected an issue with the external sensor used to monitor the outdoor temperature or flue conditions, causing it to stop functioning correctly. (As reported on Sime Format B Combi)

Pick your boiler brand for the C42 fix

Sime C42 1 model

Source

Information for the C42 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 7 July 2026