BoilerFaultCodes 66 H.....1..... High
Boiler Fault Code 66 H.....1.....
Also written as 66H1 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 66 H.....1..... appears across 1 boiler brand and 2 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 a problem with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down as a safety precaution. (As reported on Ideal Isar M30 / M100)
Pick your boiler brand for the 66 H.....1..... fix
Source
Information for the 66 H.....1..... 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