High Lockout
F.42 on Vaillant Thermocompact

Your boiler has stopped working because of a wiring issue or an electrical connection problem inside the main control panel.

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

Faulty cable loom | Faulty or incorrect loom fitted

What causes this fault?

This fault occurs when the boiler's internal computer stops receiving the correct electrical signals from the main wiring system. The most common reason is that the plastic connectors have vibrated loose over time or the wires themselves have developed a break or short circuit. Because the control board cannot safely communicate with the various parts of the boiler, it shuts down as a safety precaution.

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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