Boiler Fault Code 9F:0 to 9F:40

Also written as 9F0TO9F40 — same fault.

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

Fault code 9F:0 to 9F:40 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 is notifying you that there is a specific temperature difference between the water flowing out and returning, which usually indicates an adjustment is needed in the system's settings or water flow. (As reported on Viessmann Vitodens 300-W)