Vaillant F.3 fault codewhat it means and how to fix it
The cylinder temperature sensor circuit is open. The boiler has no reading from the probe in the hot water cylinder, so it cannot tell when to charge it and will not try.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.3
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.3 means
F.3 is the cylinder sensor reading nothing, where F.13 is the same sensor reading a short. Vaillant print it on the ecoTEC plus ..6/5-5 as "cylinder temperature sensor interruption — shift-load cylinder temperature sensor defective or not connected", checked "on the shift-load cylinder, only in conjunction with F.91" — the storage-combi and actoSTOR wording — and the check list is the temperature sensor plug, the PCB plug and the cable harness.
The 48–64 kW ecoTEC plus prints it more plainly for a system boiler with a remote cylinder: "NTC sensor defective, replace; NTC plug loose or not plugged in, check; connection to cylinder electronics defective, check". The 2022 CS/1-5 shows F.003, "the signal for the cylinder temperature sensor has been interrupted", with the wiring harness and the sensor as causes.
On a system boiler the sensor is the probe in the cylinder, wired back to the boiler over the longest sensor cable on the installation, and an open circuit anywhere on that run produces the code, which is why the harness features so heavily. The pro, ecoFIT and ecoTEC sustain tables do not carry F.3; a plain combi has no cylinder to sense.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Cylinder sensor cable broken or disconnected | The run from the boiler to the cylinder passes through cupboards and voids and is the usual failure point. A pulled plug at either end produces the same reading. | Common |
| Sensor probe failed open | The thermistor in the cylinder pocket has broken down. Vaillant's "NTC sensor defective, replace". | Common |
| PCB plug not seated | Named on the ecoTEC plus check list. A connector at the board end disturbed during other work. | Occasional |
| Cylinder electronics connection | The 48–64 kW wording, for installations where the cylinder has its own electronics module. A fault in the link between the two. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£200
Time on site
1 hour
A cylinder probe is £90 to £160 fitted; a cable run repaired or replaced to a remote cylinder can add to that.
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.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
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.3
My heating works but the cylinder is cold. Is that F.3?
It fits. With no cylinder reading the boiler will not start a cylinder charge, and many installations carry on heating regardless. The cylinder will stay cold until the circuit is repaired.
Does the engineer need to open the boiler?
Usually the cylinder end first. The probe and its cable are at the cylinder, and a meter there tells the engineer whether the fault is the sensor, the cable or something at the boiler.
Is there an immersion heater I can use meanwhile?
Most cylinders have one. If yours does, it will heat the water independently of the boiler while you wait, though it is a dearer way to do it.
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.