High Lockout
L4 on Ideal Logic Combi ESP1

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water as it leaves the unit, causing the system to shut down as a safety precaution.

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

Flow Thermistor Fault

What causes this fault?

This fault occurs when the boiler's control board can no longer get an accurate reading from the sensor that monitors hot water temperature. The most common reason is that the sensor has physically failed or developed an electrical short, though loose or damaged wiring between the sensor and the main board can also cause the connection to drop out.

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

L1View →L2View →F2View →

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