High Lockout
F.64 on Vaillant ecoTEC

Your boiler has detected an electrical fault or a sensor failure and has shut down as a safety precaution.

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

Electronics/NTC fault | Short-circuit in flow or return NTC or electronics defective

What causes this fault?

This fault occurs when the boiler's control system receives nonsensical temperature readings, such as an impossible jump in heat or a signal suggesting a short circuit. The most common reason is that one of the internal temperature sensors has failed electrically or the main control board is no longer able to process data correctly. Because the boiler cannot accurately monitor how hot the water is, it shuts down to prevent overheating and internal damage.

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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