5B:0 Medium

Viessmann 5B:0 Fault Code

Also written as 5B0 — same fault.

The Viessmann 5B:0 fault code means: your boiler has detected that the hot water cylinder is connected directly to the unit, but there is likely a communication error or a faulty sensor preventing it from heating your water correctly.. It appears on 5 Viessmann models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: DHW cylinder directly connected to the boiler

What causes the Viessmann 5B:0 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.

Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the 5B:0 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 5B:0 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026