BoilerFaultCodesVaillant F.6 Fault Code
Also written as F6 — same fault.
The Vaillant F.6 fault code means: your boiler has stopped working because a sensor that measures the temperature of the water leaving the unit has stopped sending a reliable signal.. It appears on 4 Vaillant models in our database.
Technical description: Your flow NTC has developed a fault.
What causes the Vaillant F.6 fault?
This fault occurs when the internal sensor responsible for monitoring the heat of the water leaving the boiler fails to send a consistent electrical signal to the main control board. The most common reason is that the sensor has either suffered an internal electrical failure or developed a loose connection, causing the boiler to shut down as a safety precaution. Over time, constant changes in temperature can wear these components down until they eventually fail to report accurately.
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 F.6 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.6 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