High Lockout
68 on Viessmann Vitodens 100-W

Your boiler has lost its connection to the sensor that measures the water temperature coming back into the unit, which often results in the system shutting down for safety.

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

Control mode: Return temperature sensor lead break

What causes this fault?

This fault occurs when the boiler’s internal computer loses the electrical signal from the sensor that monitors the temperature of the water returning from your radiators. The most common reason is that the sensor itself has developed an internal electrical failure or the thin wires connecting it to the main control board have become loose or damaged. Because the boiler can no longer verify if the water is at a safe temperature, it shuts down as a precautionary measure.

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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