High Lockout
50:1 on Viessmann Vitodens 050-W

Your boiler has detected a short circuit in the sensor that monitors your hot water temperature, meaning it can no longer accurately heat your tap water.

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

Constant temperature control units

What causes this fault?

This fault occurs when the electrical signal from the hot water sensor becomes constant, usually because the internal wiring of the sensor has failed or become damaged. This prevents the boiler from knowing the temperature of the water in your cylinder, causing it to stop heating your taps to prevent overheating. The most common reason is a component failure within the probe itself or water ingress affecting the electrical connections.

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

A9View →

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