Vaillant F.06 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.06 fault code mean?
The F.06 fault code on Vaillant boilers indicates that the boiler's control board is receiving an unreliable or out-of-range signal from the flow NTC thermistor — the sensor responsible for monitoring the temperature of water leaving the heat exchanger and entering your heating circuit. Without a trustworthy flow temperature reading, the boiler cannot safely regulate the burner, so it locks out to prevent overheating or damage. The fault can present as either a complete loss of signal from the sensor or a reading that falls well outside the expected operating range. On most Vaillant models the F.06 code specifically refers to the flow (outgoing) NTC rather than the return sensor, though an engineer will typically test both during diagnosis.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed or degraded flow NTC thermistor Common
NTC thermistors change their electrical resistance with temperature. Over time — particularly in older boilers or those with poor water quality — the thermistor element can degrade, crack, or fail entirely, producing readings the PCB rejects as invalid. This is the single most common root cause of F.06.
- Loose or disconnected wiring at the sensor Common
The NTC connects to the PCB via a wiring harness and push-fit connector. Vibration, previous servicing work, or simply age can cause the connector to work loose or pull away from the thermistor body, breaking the signal circuit intermittently or completely.
- Damaged wiring harness Sometimes
Chafed, pinched, or corroded wires along the harness between the flow sensor and the PCB can introduce resistance or open circuits that the boiler misinterprets as a sensor fault. Water ingress inside the casing — perhaps from a previous leak — is a common cause of harness corrosion.
- Faulty PCB Rare
If the thermistor and all associated wiring test within normal resistance and continuity values, the PCB input circuit for the sensor may have failed. This is the least likely cause but should be tested by an engineer before the board is condemned and replaced.
How to fix it
- Check your gas supply is working DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier before doing anything else. A loss of gas can sometimes cause secondary fault codes to appear.
- Check the system pressure DIY safe
Look at the pressure gauge on the boiler front panel. With the system cold it should read between 1.0 and 1.5 bar. If it has dropped below 1 bar, top it up via the filling loop and then attempt a reset. Low pressure itself does not cause F.06, but it is worth confirming the boiler is otherwise healthy before calling an engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (or follow the reset procedure in your specific model's user guide) for around three seconds. If F.06 clears and does not return within a short test run, monitor the boiler over the next day or two. If the fault reappears, do not keep resetting — repeated lockouts on the same code indicate a genuine component fault that needs engineer attention.
- Engineer inspection: test the flow NTC thermistor Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and use a multimeter to measure the resistance of the flow NTC thermistor at a known temperature. A healthy NTC has a predictable resistance-temperature curve; a reading that is open-circuit, short-circuit, or wildly off-curve confirms sensor failure and the part must be replaced.
- Engineer inspection: check wiring harness and connectors Gas Safe engineer
The engineer will inspect the connector at the thermistor body for corrosion, pushed-back pins, or moisture, then trace the harness back to the PCB checking for chafing, pinching, or broken conductors. Any damaged section of harness should be replaced rather than repaired with tape.
- Engineer inspection: test and replace PCB if required Gas Safe engineer
If the thermistor and wiring harness both test within specification, the PCB sensor input circuit is the likely culprit. The engineer will confirm this diagnosis before ordering a replacement board, as PCBs are the most expensive component in this fault tree.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the single reset did not clear the fault permanently, you need a Gas Safe registered engineer. You can find one via the Gas Safe Register website (gassaferegister.co.uk). All work inside the boiler casing — including thermistor replacement, wiring work, and PCB replacement — is legally restricted to registered engineers. Do not open the casing yourself.
Parts you may need
- Flow NTC thermistor (Vaillant OEM or equivalent) · from £20
- Wiring harness / sensor cable assembly · from £35
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.06 myself by replacing the NTC sensor?
No. Although the NTC thermistor is an inexpensive part (typically £10–£30), replacing it requires opening the boiler casing and working on internal components. Under UK law, only Gas Safe registered engineers are permitted to carry out work inside a gas appliance. Attempting it yourself would be illegal, could void any remaining warranty, and may be unsafe. The only safe DIY steps are checking your gas supply, verifying system pressure, and attempting a single reset.
How much does it cost to fix a Vaillant F.06 fault in the UK?
For most households the repair involves replacing the flow NTC thermistor and possibly a section of wiring harness. Including parts and labour, this typically costs between £120 and £320. An engineer call-out or first-hour charge is usually £80–£120 depending on your region, and the thermistor part itself is only £10–£30. If the PCB turns out to be the root cause, costs rise considerably — a replacement Vaillant PCB can run to £350–£500 or more including fitting — but this is the least common outcome and a good engineer will confirm the diagnosis before recommending it.
Why does my Vaillant boiler keep showing F.06 after I reset it?
A fault that keeps returning after a reset is telling you that the underlying problem has not been resolved. Each reset simply clears the lockout temporarily; if the flow NTC is sending bad signals the boiler will lock out again within minutes or hours. Repeatedly resetting without fixing the root cause will not help and may complicate the engineer's diagnosis. One reset to test whether the fault was transient is reasonable — after that, call a Gas Safe engineer.
Will F.06 affect my hot water as well as my heating?
Yes. Because the Vaillant F.06 fault triggers a full safety lockout, both central heating and domestic hot water will be unavailable until the fault is resolved. The boiler will not fire at all while it is in lockout, regardless of whether you are calling for heating or hot water.