L5 or F5 High

Ideal L5 or F5 Fault Code

Also written as L5ORF5 — same fault.

The Ideal L5 or F5 fault code means: your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators, preventing it from heating correctly.. It appears on 13 Ideal models 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: Return thermistor fault
Typical repair cost £120-£200 (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 L5 or F5 fault code

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

Source

Ideal L5 or F5 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 31 August 2026