High Lockout
5b:0 on Viessmann Vitodens 300-W

Your boiler has detected that your hot water cylinder is connected directly to the unit, but there is likely a communication error or a missing sensor signal prevent it from heating your water.

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

DHW cylinder directly connected to the boiler

What causes this fault?

This fault occurs when the boiler loses its connection to the temperature sensor inside your hot water tank. The most common reason is a faulty sensor or a loose wire in the cable harness, meaning the boiler can no longer tell how hot your water is and stops heating it 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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