High Lockout
9C on Viessmann Vitodens 200-W

Your boiler is unable to read the temperature of your hot water cylinder because the connection to the sensor has been lost or damaged.

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 loses its electrical connection to the temperature sensor attached to your hot water tank. The most common reason is that the thin wiring has become loose, corroded, or accidentally damaged, preventing the boiler from knowing if your water is hot enough. Because the control board can no longer monitor the safety of the cylinder, it stops providing hot water 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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Related fault codes

9BView →A7View →9FView →

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