High Lockout
F5 on Ideal Logic Combi C24 C30 C35

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

What causes this fault?

This fault occurs when the boiler's control board receives an incorrect or missing signal from the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be caused by loose electrical connections or a fault on the main control board.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F4View →F3View →L2View →

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