52:1 Medium

Viessmann 52:1 Fault Code

Also written as 521 — same fault.

The Viessmann 52:1 fault code means: your boiler is struggling to accurately measure the water temperature in a specific part of your heating system, which may cause your heating to run inefficiently or not at all.. 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: With flow temperature sensor for low loss header (automatic adjustment upon recognition)

What causes the Viessmann 52:1 fault?

This fault occurs when the boiler loses communication with the temperature sensor located on the low loss header, which is a component that balances water flow in larger or complex heating systems. The most common reason is a faulty sensor that has failed electrically or a loose connection in the wiring that prevents the boiler from monitoring the system's true temperature.

Typical repair cost £120-220 (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 52:1 fault code

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

Source

Viessmann 52:1 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