BoilerFaultCodesZanussi L4 Fault Code
The Zanussi L4 fault code means: your boiler has detected a problem with the internal temperature sensor that monitors the water leaving the unit, causing it to shut down as a safety precaution.. It appears on 1 Zanussi model in our database.
Technical description: Flow Thermistor Fault
What causes the Zanussi L4 fault?
This fault occurs when the sensor responsible for measuring the heat of the water leaving your boiler fails to send a correct signal to the main control board. The most common reason is that the sensor has physically degraded or developed an electrical short, but it can also be caused by loose wiring or a glitch in the boiler's internal computer.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Zanussi models that show the L4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Zanussi L4 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