Vaillant F.0 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.0
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 does the boiler do when you ask for heat?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Nothing happens at all — no fan, no ignition attempt
Expected behaviour. The boiler will not begin a cycle it cannot monitor, so it stays inert rather than attempting ignition.
Book a Gas Safe registered engineer. There is no hazard in leaving it switched off in the meantime.
It sometimes runs normally for hours before the code returns
Intermittent means a connection rather than a dead sensor. A cracked conductor or a marginal plug opens and closes as the appliance expands and contracts.
Ask for the harness and connectors to be checked before any part is ordered. This is where money gets wasted.
It began immediately after a circuit board was replaced
Almost certainly a lead on the wrong terminal or not reconnected. The sensor itself is very unlikely to have failed on the same day.
Return to the engineer who fitted the board.
What F.0 means
F.0 means the flow temperature sensor has been interrupted — an open circuit somewhere between the sensor and the printed circuit board. The flow sensor is the boiler's primary reference for everything it does with the burner: it decides how hard to fire, when to modulate down, and when to stop. Without that reading the appliance has no safe way to regulate output, so it refuses to light at all.
The important nuance is that F.0 describes a broken connection rather than a wrong temperature. The circuit is electrically open, which is why the causes are so often mechanical: a plug that has vibrated off its pins, a cable harness chafed through where it crosses an edge, or a sensor that has failed open with age. Vaillant documentation lists all of these ahead of board failure.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor plug unseated at the board | Years of heating and cooling work a connector loose until one pin no longer makes contact. | Common |
| Sensor failed open circuit | These thermistors commonly fail towards infinite resistance, which the board reads as no sensor present at all. | Common |
| Break in the cable harness | A conductor fractures inside intact-looking insulation, often where the loom is bent tightly around a bracket. | Occasional |
| Wrong connection on the circuit board | After a board change a sensor lead put on an adjacent terminal presents as an interrupted flow sensor. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£480
Flow sensor £110 to £190 fitted. Connector or harness repair often within a call-out of £80 to £150. Circuit board, where testing points to it, £300 to £480.
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.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
- 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.0
Is F.0 the same as F.10?
They are the two halves of the same failure. F.0 is the flow sensor circuit interrupted, an open circuit. F.10 is the flow sensor short circuited, the opposite electrical condition. Both mean the boiler has lost a usable reading, but the wiring fault behind each is different, so it is worth quoting the right one.
Is it safe to leave the boiler off?
Yes. There is no gas or combustion risk here — the appliance has declined to light. In cold weather bear frost protection in mind, since a boiler that cannot run cannot protect your pipework either.
Will I need a new circuit board?
Usually not. The sensor, its plug and its cable account for the large majority of F.0 faults, and all three are inexpensive. A board is considered only after the sensor has been measured and the wiring proven sound.
How much is this likely to be?
One of the more affordable Vaillant faults. Expect £110 to £190 for a sensor fitted, or a call-out charge alone if reseating a plug turns out to be the whole job.
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 support portal
- 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.