High Lockout
F.91 on Vaillant ecoTEC Plus 635

Your boiler has lost the ability to monitor the water temperature in your hot water cylinder due to an electrical connection fault.

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

Short circuit: Cylinder temperature sensor. NTC sensor defective, short circuit in the cable harness, cable/housing

What causes this fault?

This fault occurs when the electrical signal from the hot water cylinder’s temperature probe is interrupted or corrupted. The most common reason is that the internal wiring of the sensor has failed or become damaged, causing a short circuit that prevents the boiler from knowing how hot your water is. In some cases, moisture ingress or loose connections at the sensor housing are the physical culprits.

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.11View →

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