High Lockout
F5 on Ideal Logic Max Plus

Your boiler has detected a sensor problem that means it cannot accurately measure 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 internal sensor responsible for monitoring the temperature of the water coming back into the boiler fails or provides an Impossible reading. The most common reason is that the sensor has physically degraded over time or a loose electrical connection is preventing it from communicating with 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 →L4View →L2View →

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