F 4 L 4 High

Ideal F 4 L 4 Fault Code

Also written as F4L4 — same fault.

The Ideal F 4 L 4 fault code means: your boiler has detected a problem with the internal sensor that monitors the temperature of the water as it leaves the unit, causing it to shut down as a precaution.. It appears on 1 Ideal 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
Typical repair cost £90-160 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Ideal models that show the F 4 L 4 fault code

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

Source

Ideal F 4 L 4 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026