High Lockout
9C on Viessmann Vitodens 333

Your boiler has lost connection with the sensor that monitors the temperature of your hot water cylinder, meaning it cannot heat your water 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

Control mode - Cylinder temperature sensor lead break

What causes this fault?

This fault occurs when the boiler's control board can no longer detect a signal from the probe that monitors your hot water tank. The most common reason is that the internal wiring has become damaged or loose, or the sensor itself has suffered an electrical failure due to age. Because the boiler doesn't know how hot the water is, it stops heating the cylinder as a safety precaution.

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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