High Lockout
Return Thermistor Fault on Ideal Logic Max Heat

Your boiler has detected an issue with a temperature sensor that monitors the water returning from your radiators, preventing the system from heating up safely.

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

Wiring or resistance issues with the Return Thermistor located in the pump housing (upper LHS of boiler).

What causes this fault?

This fault occurs when the boiler's control board receives an incorrect or missing signal from the sensor that measures the temperature of the water coming back from your radiators. The most common reason is that the internal sensor has developed a physical defect or the electrical connection has become loose or corroded over time.

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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