BoilerFaultCodes C 20 High
Boiler Fault Code C 20
Also written as C20 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code C 20 appears across 1 boiler brand and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is currently unable to detect the flow of water because the sensor responsible for monitoring it is providing an incorrect reading or has stopped communicating with the system, which will prevent your heating and hot water from working. (As reported on Atag S 25CR)
Pick your boiler brand for the C 20 fix
Source
Information for the C 20 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