High Lockout
51 on Viessmann Vitodens 333

Your boiler is unable to provide hot water because of a faulty internal sensor that has stopped communicating correctly.

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

No DHW heating - Cylinder temperature sensor 2/outlet sensor shorted out

What causes this fault?

This fault occurs when the internal electronic probe responsible for monitoring the hot water temperature develops an electrical short circuit. The boiler’s control board can no longer read the water temperature safely, so it disables the hot water function to prevent overheating. This is typically caused by natural wear and tear of the sensor or moisture ingress damaging the electrical connections over time.

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

59View →A7View →

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