High Lockout
C4237 on Worcester Bosch CDi Classic

Your boiler's main internal computer has developed a serious electronic fault and can no longer control the heating process 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

Control box or heat control module (HCM) is defective

What causes this fault?

This fault occurs when the boiler's internal computer chips suffer a critical communication failure or hardware burnout. The most common reason is age-related degradation of the electrical components, though it can also be triggered by a sudden power surge or moisture getting into the control box casing.

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

C6View →

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