Your flow NTC (temperature sensor) has developed a fault
The F.6 error code on Vaillant boilers indicates your flow ntc (temperature sensor) has developed a fault.
Vaillant F.6 means: your flow ntc (temperature sensor) has developed a fault. On this component the documented failure modes are the temperature sensor has failed — short circuit or open circuit or wiring, connector or harness of the temperature sensor damaged or loose. You can safely reset the boiler once — if F.6 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does F.6 Mean
The F.6 code is shown by the boiler’s control board when it detects your flow ntc (temperature sensor) has developed a fault. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F.6 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 8 different manufacturers.
| Cause |
|---|
| The temperature sensor has failed — short circuit or open circuit |
| Wiring, connector or harness of the temperature sensor damaged or loose |
Safety Warning
F.6 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
How to Fix It
Measure the resistance of the temperature sensor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Unplug and re-seat the connector of the temperature sensor, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Vaillant service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Vaillant models that can display fault code F.6:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vaillant models | — | Possible |
Frequently Asked Questions
Measure the resistance of the temperature sensor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range. According to the technical documentation, this is the first check on the list for F.6.
F.6 is not classified as a high-severity fault, so the boiler can be reset once. If F.6 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.