High Lockout
F.1 on Vaillant ecoTEC Plus 840

Your boiler has stopped working because it cannot accurately measure the temperature of the water returning to the unit due to a faulty sensor or wiring.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

Return–NTC open circuit. NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor has physically failed or a wire has shaken loose, meaning the boiler can no longer safely regulate its heat output. Because the system doesn't know how hot the water is, it shuts down immediately to prevent internal 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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Related fault codes

F.0View →F.10View →F.11View →

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