High Lockout
FA on Ideal Logic Max Combi

Your boiler has detected that the temperature sensors for the water going in and out are reporting incorrect or swapped readings, which has caused the system 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

NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT

What causes this fault?

This fault occurs when the boiler detects that the water pipe leaving the unit is cooler than the pipe returning to it, which is physically impossible during normal operation. The most common reason is that the internal temperature sensors have failed or their wiring has become cross-connected, leading the boiler to shut down to prevent overheating. It can also be triggered by a complete lack of water circulation caused by a seized pump or a significant blockage in the system.

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

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