Vaillant F.13 fault codewhat it means and how to fix it
The cylinder or warm-start temperature sensor has short-circuited. On a system boiler that is the sensor in your hot water cylinder; on a combi it is the warm-start sensor.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.13
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What F.13 means
F.13 is the one sensor code whose sensor may not be in the boiler at all. Vaillant's ecoTEC plus table reads "cylinder temperature sensor short circuit — warm-start sensor/cylinder temperature sensor defective or not connected", and that pairing is the key to it. On a system boiler the cylinder sensor is the probe in the hot water cylinder, often several metres away, wired back to the boiler; a short circuit anywhere along that run produces F.13.
On a combi the same input is used for the warm-start sensor that keeps the secondary heat exchanger primed, and on a storage combi with actoSTOR it is the shift-load cylinder's sensor, checked "in conjunction with F.91".
The check list is the NTC plug, the earth connection, the cable harness, the sensor and the PCB. The 2005 servicing instructions call it "tank NTC short circuit", the ecoTEC sustain marks its row "applies to: product with heating mode only", and the 2022 CS/1-5 shows F.013 with the wiring and the sensor as causes. Because the cylinder lead is the longest sensor cable on the installation, the harness is a more likely culprit here than on the flow and return codes.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Cylinder sensor cable damaged | The lead to a remote cylinder runs through an airing cupboard and behind panels; crushed, trapped or rodent-damaged cable is the common finding on a system boiler. | Common |
| Sensor probe failed | The cylinder probe or the combi's warm-start sensor has broken down internally and reads as a short. Vaillant list the sensor itself on every version of the row. | Common |
| Earth connection fault | Named on the ecoTEC plus check list. A poor earth can make a healthy sensor read as shorted to the casing, and it is checked before any part is changed. | Occasional |
| Board input fault | The PCB is on the list and considered last, after the probe and the full length of its cable have been measured. | 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 repair to a remote cylinder can add to that. Within-boiler sensor work is £110 to £200.
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
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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
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.13
My heating still works. Why is hot water the only thing affected?
Because the sensor only tells the boiler about the cylinder. Some installations keep heating with F.13 present and simply stop charging the cylinder; others lock out for both. Either way the cylinder will not be reheated until the circuit is repaired.
Does an engineer need to open the boiler for this?
Often not first. On a system boiler the probe and its cable are at the cylinder, and that is where an engineer normally starts; the boiler end is checked if the cylinder end tests sound.
I have a combi with no cylinder. Why am I seeing F.13?
On a combi the same input serves the warm-start sensor inside the boiler, so F.13 is still a sensor fault — just a different sensor. The engineer works from Vaillant's same check list.
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.