High Lockout
20 on Viessmann Vitodens 333

Your boiler's internal computer has detected a faulty or damaged temperature sensor, preventing the system from accurately measuring how hot the water is.

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

Controls without flow temperature sensor (low loss header) - System flow temperature sensor shorted out

What causes this fault?

This fault occurs when the electrical signal from the temperature sensor is interrupted or compromised, usually due to an internal short circuit. The most common reason is that the sensor's internal components have failed over time or moisture has seeped into the wiring connections. Because the boiler can no longer safely monitor how hot the water is, it shuts down 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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