F.545 Medium

Viessmann F.545 Fault Code

Also written as F545 — same fault.

The Viessmann F.545 fault code means: a sensor that monitors the temperature for your second heating zone (such as underfloor heating) has developed an electrical fault, causing your boiler to close the mixing valve to prevent overheating.. It appears on 4 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: Mixer closes. Heating circuit pump is operational. Short circuit, flow temperature sensor, heating circuit 2 with mixer.
Typical repair cost £150-250 (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 F.545 fault code

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

Source

Viessmann F.545 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 7 July 2026