High Lockout
F.71 on Vaillant ecoTEC Plus 840

Your boiler is struggling to accurately measure the water temperature leaving the unit, usually because a sensor has failed or become disconnected.

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

Flow NTC reports constant value (stuck at)

What causes this fault?

This fault occurs when the boiler's control board detects that the temperature sensors are giving constant or illogical readings that don't change as the water heats up. The most common reason is that the flow sensor has either physically failed internally or has lost its electrical connection to the main circuit board. Without accurate temperature data, the boiler shuts down as a safety precaution to prevent overheating.

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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