Vaillant F.10 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.10 fault code mean?
The F.10 code on a Vaillant boiler indicates that the heating flow NTC thermistor — the sensor that monitors the temperature of water leaving the heat exchanger on its way out to your radiators — has reported a short circuit. Because the boiler cannot obtain a reliable flow temperature reading, it locks out as a safety precaution. This is the flow (outgoing) sensor, not the return sensor. The fault will not clear permanently until the underlying electrical or sensor issue is resolved; a reset may temporarily clear the display, but the code will almost certainly return if the root cause remains.
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
- Faulty or shorted flow NTC thermistor Common
The NTC thermistor itself has developed an internal short circuit or its resistance has drifted outside the acceptable range. A healthy sensor should read approximately 12 kΩ at 20°C. If the reading is near zero or wildly out of spec, the sensor has failed and needs replacing.
- NTC plug loose, disconnected, or shorted against the casing Common
The connector linking the NTC sensor to the wiring loom can work loose over time due to vibration, or it may have been disturbed during a recent service. In some cases the plug has shifted and made contact with the metal boiler casing, creating a short to earth and triggering F.10.
- Damaged or chafed wiring harness Sometimes
The cable running between the flow NTC and the main PCB can become pinched, chafed against sheet metalwork, or degraded by heat or moisture. A break or short anywhere along this loom will produce the same fault code as a failed sensor.
- Faulty or water-damaged main PCB Rare
If the sensor and wiring both test correctly, the PCB's NTC input circuit may itself be damaged — often caused by a water leak tracking along a wiring harness onto the board. This is a less common but more expensive root cause.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (or follow the reset procedure in your Vaillant user guide) for the model you have. A single reset can clear a one-off electronic glitch. If the F.10 code disappears and does not return within a normal heating cycle, monitor the boiler over the next 24 hours. Do not reset more than two or three times in total.
- Check your gas supply DIY safe
Confirm that other gas appliances in the property (cooker, gas fire) are working normally and that your gas meter's emergency control valve is fully open. A loss of gas supply can sometimes cause boilers to lock out, though it would normally produce a different code.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler — it should sit between 1.0 and 1.5 bar when the system is cold. If it has dropped below 0.5 bar, top it up via the filling loop to around 1.2 bar. Low pressure alone will not cause F.10, but it is worth ruling out before calling an engineer.
- Do not remove the boiler casing or touch internal wiring Gas Safe engineer
All internal inspection — checking the NTC resistance, examining the wiring loom, testing PCB connections — must be carried out by a Gas Safe registered engineer. Removing the boiler casing or probing internal components is not safe or legal for an unregistered person.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
The engineer will use a multimeter to measure the flow NTC resistance (expected ~12 kΩ at 20°C), inspect the full wiring harness for damage or short circuits, check connector seating at the PCB, and replace whichever component is at fault — whether that is the NTC sensor, the wiring loom, or in rare cases the PCB itself. Ask the engineer to check for any moisture ingress that may have caused or accelerated the fault.
Parts you may need
- Vaillant flow NTC thermistor (heating flow temperature sensor) · from £15
- NTC sensor wiring harness / loom · from £35
- Vaillant main PCB (model-specific) · 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 £90–£280, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting my Vaillant boiler to clear the F.10 code?
A reset might temporarily clear the display if the fault was caused by a fleeting glitch, but if F.10 returns after one or two resets, the underlying sensor or wiring fault is still present. Repeatedly resetting the boiler will not fix a shorted NTC or damaged loom, and doing so too many times can cause additional wear. If the code comes back, stop resetting and book an engineer.
How much does it cost to fix a Vaillant F.10 fault in the UK?
For most homeowners, the repair involves replacing the flow NTC sensor and/or repairing the wiring harness, which is a straightforward job. Expect to pay roughly £90–£280 in total including parts and labour, depending on your location and the engineer's call-out rate. London and South East engineers typically charge more than those in other regions. If the PCB also turns out to be damaged, the cost can rise significantly — a replacement Vaillant PCB alone can be £180–£400 before labour — but PCB failure is a less common cause of F.10.
What is an NTC thermistor and why does it cause the boiler to lock out?
NTC stands for Negative Temperature Coefficient — a type of resistor whose electrical resistance decreases predictably as temperature rises. Your boiler's control board reads this changing resistance to calculate the water temperature. When the NTC short-circuits, its resistance drops to near zero, which the PCB interprets as an impossibly high temperature reading (or simply an invalid signal). To prevent the boiler running in an uncontrolled state, it locks out and displays F.10 until the fault is corrected.
Could a Vaillant F.10 fault be linked to a water leak inside the boiler?
Yes — this is worth asking your engineer to check. If a small internal leak has allowed water to track along the NTC wiring harness and onto the PCB connector, it can cause the short circuit that triggers F.10. Simply replacing the sensor without finding and fixing the leak source would likely result in the fault returning. An engineer should inspect the heat exchanger, pump seals, and any other internal joints as part of a thorough diagnosis.