Boiler Fault Code 9C:20

Also written as 9C20 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026

Fault code 9C:20 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has lost communication with one of its internal control modules or an external smart accessory, causing it to use temporary backup settings. (As reported on Viessmann Vitodens 200)