High Lockout
9B on Viessmann Vitodens 333

Your boiler has lost connection with its internal hot water storage sensor and cannot currently heat your water effectively or safely.

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 - Sensor S3 lead break

What causes this fault?

This fault occurs when the boiler's control board can no longer communicate with the sensor responsible for monitoring the temperature of your hot water. The most common cause is a physical failure of the internal sensor probe or a loose connection in the wiring harness that links the sensor to the main computer. Without this signal, the boiler cannot safely determine how hot the water is, so it stops heating the cylinder 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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