BoilerFaultCodesVaillant F6 Fault Code
The Vaillant F6 fault code means: your boiler has lost its ability to measure the temperature of the water leaving the unit, usually due to a faulty sensor or a wiring issue.. It appears on 2 Vaillant models in our database.
Technical description: Central heating flow NTC fault
What causes the Vaillant F6 fault?
This fault occurs when the boiler's internal computer can no longer detect a reliable signal from the sensor that monitors the temperature of the water being sent to your radiators. The most common reason is that the sensor itself has failed internally due to age, or the electrical connection between the sensor and the main control board has become loose or damaged.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vaillant models that show the F6 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F6 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