How much does it cost to fix Vaillant error code F.85?

Typical repair cost for the Vaillant F.85 fault is £100-£180 including labour.

F.85 High

Vaillant F.85 Fault Code

Q: What does the Vaillant F.85 fault code mean?

Your boiler has detected that the temperature sensors are giving unusual readings, likely because they are loose or have been installed on the wrong pipes.

Also written as F85 — same fault.

The Vaillant F.85 fault code means: your boiler has detected that the temperature sensors are giving unusual readings, likely because they are loose or have been installed on the wrong pipes.. It appears on 34 Vaillant models in our database.

Technical description: Fault: NTCs fitted incorrectly Flow/return temperature sensors returning incorrect/implausible values. Check: Flow/return temperature sensor (correct thermal connection).

What causes the Vaillant F.85 fault?

This fault occurs when the boiler's control board detects that the temperature sensors are not sending consistent or logical readings. The most common reason is that the sensors have vibrated loose from the internal pipes or were accidentally clipped onto the wrong ones during a recent repair. Because the boiler cannot accurately track how hot the water is, it shuts down for safety.

Typical repair cost £100-£180 (including labour)

Engineer required

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

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Vaillant models that show the F.85 fault code

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

Source

Vaillant F.85 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

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