Medium Lockout
PCB 38 CH light flashing on Ideal Classic FF

Your boiler has detected a communication issue or a component failure within its central heating temperature sensing system, meaning it cannot accurately monitor the water temperature for your radiators.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Check the CH sensor and panel control and/or the wires connecting them to the PCB (indicates a fault with the CH sensor or its connections).

What causes this fault?

This fault occurs when the boiler's main computer loses contact with the sensor that monitors the temperature of the water heading to your radiators. It is usually caused by the internal sensor failing due to age or a loose wire connection preventing the signal from reaching the control board. Because the boiler cannot tell how hot the water is, it stops heating as a safety precaution to prevent overheating.

This issue often surfaces in autumn when the central heating is turned back on after months of disuse, putting sudden electrical demand on aging sensors.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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