High Lockout
F.78 on Vaillant ecoTEC Pro 24

Your boiler is confused because it is looking for a hot water sensor that is either missing or has a broken connection, preventing it from heating your water correctly.

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

Interrupt.: DHW outlet sensor on ext. contr. - UK link box is connected without the domestic hot water temperature sensor being bridged

What causes this fault?

This fault occurs when the boiler’s internal control system expects to see a hot water sensor connected but cannot find a signal. It is usually caused by a loose connection in the wiring link box or a faulty temperature sensor that has stopped communicating with the main circuit board. Without this data, the boiler shuts down as a safety precaution because it cannot accurately monitor the temperature of the water being sent to your taps.

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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