High Lockout
FA on Ideal Logic Combi C24 C30 C35

Your boiler has detected that the water temperature sensors are giving conflicting readings, often caused by the flow and return pipes being switched or a sensor failing.

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 returning from your radiators is hotter than the water leaving the boiler, which is physically impossible under normal conditions. The most common reasons are that the flow and return pipes were crossed during installation, or one of the internal temperature sensors has developed a fault and is sending the wrong signal to the control board.

This fault often appears immediately after summer if homeowners have had new radiators or a new boiler installed while the heating was turned off.

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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