BoilerFaultCodesZanussi FA Fault Code
The Zanussi FA fault code means: your boiler has detected that the water temperature sensors are giving conflicting readings, often caused by the flow and return pipes being switched or a sensor failing.. It appears on 1 Zanussi model in our database.
Technical description: NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT
What causes the Zanussi FA fault?
This fault occurs when the boiler detects that the water returning from your radiators is hotter than the water leaving the boiler, which is physically impossible under normal conditions. The most common reasons are that the flow and return pipes were crossed during installation, or one of the internal temperature sensors has developed a fault and is sending the wrong signal to the control board.
This fault often appears immediately after summer if homeowners have had new radiators or a new boiler installed while the heating was turned off.
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 FA fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Zanussi FA 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