BoilerFaultCodesZanussi FU Fault Code
The Zanussi FU fault code means: your boiler has detected that the water is heating up too quickly or isn't moving through the system properly, causing it to overheat and shut down for safety.. It appears on 1 Zanussi model in our database.
Technical description: DIFF GREATER THAN 50°C CHECK ISOLATION VALVES ARE OPEN CHECK PUMP CHECK RAD VALVES OPEN CHECK SYSTEM BLOCKAGES
What causes the Zanussi FU fault?
This fault happens when heat cannot escape the boiler quickly enough, causing the internal temperature to spike dangerously fast. It is usually caused by a pump failure that stops water from moving or a blockage in the pipes, such as a buildup of sludge or closed valves, that prevents circulation. Effectively, the boiler is producing heat but has nowhere to send it, so it shuts down to prevent damage.
This issue often appears when the heating is turned back on for autumn, as pumps can seize up after being idle throughout the summer.
DIY fix possible
Check that at least two or three radiator valves are fully open to allow water flow.Ensure the large blue and red isolation valves underneath the boiler are in the 'open' position.Try resetting the boiler by turning the control knob to 'Reset' and then back to the original setting.
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Zanussi models that show the FU fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Zanussi FU fault code information on this page is sourced and cross-checked against the official Zanussi service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026