Boiler Fault Code 4 / U

Also written as 4U — same fault.

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

Fault code 4 / U appears across 1 boiler brand and 1 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 the ability to accurately measure the temperature of the water circulating inside it, so it has shut down as a precaution to prevent overheating. (As reported on Ideal Maximiser SE)