Vaillant F.71 fault codewhat it means and how to fix it
The 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.
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What F.71 means
F.71 is not an open circuit or a short. Those are F.0 and F.10. This is a sensor that is electrically fine and still wrong: the ecoTEC plus table says "flow temperature sensor returns implausible values", the ecoFIT, ecoTEC sustain and exclusive say it is "signalling a constant value", and the 2005 servicing instructions put it most plainly of all — "flow NTC reports constant value (stuck at)". A flow sensor clipped to the pipe leaving the heat exchanger should climb steadily once the burner lights.
One that reads the same number regardless is either not touching the pipe any more or has failed in a way that leaves its output frozen. Vaillant order the causes accordingly: incorrectly positioned on the flow pipe first, defective second, and the only instruction to the engineer on the ecoTEC plus is to check the sensor's "correct thermal connection". The 2022 CS/1-5 keeps the same two causes under F.071. This is a code that follows work on the boiler more often than it appears from nowhere, because the clip-on sensor is easy to knock off its pipe when the case is off.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor knocked off or loose on the flow pipe | Vaillant's first cause on every current table. A clip-on NTC that has come away from the pipe reads room temperature and stays there while the water behind it heats. | Common |
| Sensor failed with a frozen output | The thermistor still measures a resistance but no longer responds to heat. Vaillant's second cause and the one that needs a part. | Common |
| Poor thermal contact through scale or paste | A sensor that is on the pipe but insulated from it by debris or dried-out compound lags so far behind the water that the board rejects the reading. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
40 minutes
Re-seating a sensor is a call-out charge of £80 to £120. A replacement flow sensor is £110 to £160 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
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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.10Get it checkedThe 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.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
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Related guides
Common questions about Vaillant F.71
It started the day after a service. Related?
Very likely. The flow sensor sits where hands go during a service, and a clip that was not pushed fully home comes off with the first thermal cycle. Ring whoever did the work first; it is a two-minute correction for them.
How is F.71 different from F.0?
F.0 means the sensor circuit is broken and the board reads nothing. F.71 means the board reads a value but the value does not behave like a temperature — it sits still when it should be rising. Different fault, same sensor.
Could the boiler overheat with this fault?
It is exactly the risk the code exists to prevent. A stuck flow reading would blind the temperature limiter, so the board stops the burner rather than trust it. The lockout is protective, not a nuisance.
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
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.