How much does it cost to fix Vaillant error code F.20?

Typical repair cost for the Vaillant F.20 fault is £120-£280 including labour.

F.20 High

Vaillant F.20 Fault Code

Q: What does the Vaillant F.20 fault code mean?

Your boiler has shut down because it has detected an unsafe increase in internal temperature, likely caused by a sensor fault or an issue with water flow.

Also written as F20 — same fault.

The Vaillant F.20 fault code means: your boiler has shut down because it has detected an unsafe increase in internal temperature, likely caused by a sensor fault or an issue with water flow.. It appears on 46 Vaillant models in our database.

Technical description: Safetyswitch-off:Temperaturelimiter | Incorrectearthconnectionbetweencableharnessandproduct,flowor returnNTCdefective(looseconnection),straysparkviaignitioncable, ignitionplugorignitionelectrode

What causes the Vaillant F.20 fault?

This fault occurs because the boiler's internal sensors have detected that the water temperature has risen too quickly or exceeded a safe limit. It is usually caused by a lack of water flow, often due to a pump failure or a blockage, which prevents the heat from being carried away from the heat exchanger. In some cases, the sensor itself may have become detached or faulty, sending an incorrect reading to the control board.

This fault is more common in autumn when heating systems are first turned on after summer, as circulation pumps can seize due to months of inactivity.

Typical repair cost £120-£280 (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.20 fault code

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

Source

Vaillant F.20 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 7 July 2026