Vaillant F.10 fault codewhat it means and how to fix it
The flow temperature sensor has short-circuited. The boiler is reading an impossible value from the sensor on its outlet pipe and will not fire on it.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.10
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.10 means
Where F.0 is the flow sensor reading nothing, F.10 is it reading everything: Vaillant print the code as "flow temperature sensor short circuit", and the ecoTEC plus check list is the NTC plug, the cable harness, the cable or housing, the PCB and the sensor itself. An NTC is a resistor whose value falls as it warms; a short circuit drops that resistance to nothing, which the board would otherwise interpret as a temperature far beyond anything the heat exchanger could reach.
The 2005 servicing instructions put a figure on it — "short circuit flow NTC (< 130 °C)" — meaning the board rejects any reading that implies more than 130 °C rather than acting on it. The most instructive item on Vaillant's list is "cable/housing", the short circuit to the casing: a harness that has chafed through against the metalwork touches earth and produces exactly this code, and it is why an engineer measures the harness before fitting a sensor. On the ecoMAX and turboMAX plus the same row reads "NTC plug shorted to casing, NTC defective", and the 2022 CS/1-5 shows it as F.010.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Harness shorted to the casing | Vaillant's "cable/housing" cause. Insulation worn through where the sensor lead passes the metalwork puts the signal to earth, and the board reads a short. | Common |
| Flow sensor failed short | The thermistor has broken down internally. It is the same clip-on part as the F.0 fault and equally cheap; the diagnosis is a resistance reading. | Common |
| Water in the sensor plug | A weep from a nearby joint bridging the two pins reads as a short circuit and clears when the plug dries — which is why the code can come and go. | Occasional |
| Electronics fault | The board's input stage has failed. Vaillant list the PCB on the check list and an engineer reaches it by elimination. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£170
Time on site
45 minutes
A harness repair or plug dry-out is usually within a call-out of £80 to £120. A replacement flow sensor is £110 to £170 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant flow, return or cylinder temperature sensor (NTC)
£12–£38Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.0Get it checkedThe flow temperature sensor circuit is broken. The boiler cannot read how hot the water leaving it is, so it has stopped rather than fire blind.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors are reporting a temperature difference that cannot be real. The boiler is blaming the sensors themselves rather than the water.Related: temperature sensors
- F.1Get it checkedThe return temperature sensor circuit is open: the boiler is getting no reading from the sensor on the pipe bringing water back in, so it has stopped.Related: temperature sensors
- F.11Get it checkedThe return temperature sensor has short-circuited. Vaillant treat it as a wiring or sensor fault, and the boiler stays off until the circuit reads correctly again.Related: temperature sensors
- F.84Get it checkedThe flow and return sensors disagree in a way that cannot be true — the return reading warmer than the flow. Vaillant's first explanation is that the two sensors have been swapped.Related: temperature sensors
- F.71Get it checkedThe flow sensor is returning a value the boiler does not believe — typically one that never changes. Vaillant's first suspect is a sensor that has slipped off the pipe.Related: temperature sensors
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.10
The boiler was fine until it was serviced. Is that a coincidence?
Not necessarily. The flow sensor lead runs close to parts that are moved during a service, and a lead trapped against the casing when the case went back on is a textbook cause. Tell the engineer when it started.
Does F.10 mean the water is too hot?
No. It means the sensor is reporting a value the board knows cannot be real, so the reading is rejected. Genuine overheating is F.20, and an implausible reading that is still within range is F.71.
Will a reset clear it?
Only if the short was momentary — moisture that has since dried, for example. A sensor or harness that has failed reads short every time the board looks, and the code returns as soon as the boiler tries to run.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.