F4 Or L4 High

Zanussi F4 Or L4 Fault Code

Also written as F4ORL4 — same fault.

The Zanussi F4 Or L4 fault code means: your boiler has stopped working because it is having trouble reading the temperature of the water flowing through it, so it has safely shut down to prevent overheating.. It appears on 1 Zanussi model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Flow Thermistor Fault
Typical repair cost £120-220 (including labour)

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 F4 Or L4 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Zanussi F4 Or 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