BoilerFaultCodesYour boiler's main internal computer has developed a serious electronic fault and can no longer control the heating process safely.
Book a Gas Safe engineer within 24–48 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 fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like C4237 — no call-out fees, no surprises.
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