Boiler Fault Code Safety thermostat lockout

Also written as SAFETYTHERMOSTATLOCKOUT — same fault.

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

Fault code Safety thermostat lockout appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has shut down because it reached an internal temperature that is too high, often caused by a circulation issue or a lack of water flow. (As reported on Biasi Riva Compact 24S)