High Lockout
L5 on Ideal Logic Combi ESP1

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, causing it to shut down 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 internal computer receives an unlikely or out-of-range signal from the sensor that monitors water coming back from your radiators. The most common reason is that the sensor has physically failed or developed a loose connection, meaning the boiler can no longer safely regulate its temperature.

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

F5View →L2View →F2View →

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