Vaillant F.12 fault codewhat it means and how to fix it
The hot water outlet sensor has short-circuited. A combi or storage-combi code: heating may continue, but the boiler cannot control the hot water temperature.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.12
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.12 means
F.12 belongs to boilers that make hot water themselves. Vaillant print it on the ecoTEC plus as "DHW outlet sensor short circuit — cylinder charging sensor defective or has short-circuited", and the ecoTEC exclusive as "short circuit: DHW outlet sensor". The sensor reads the temperature of the water leaving the secondary heat exchanger towards your taps, and without a believable reading the boiler cannot modulate to the temperature you set.
Two details in the tables are worth knowing. First, both the plus and the exclusive mark the row "only in conjunction with F.91", the actoSTOR sensor fault, which tells you the outlet sensor in question on those models is part of the storage-combi arrangement — a VUI storage combi or an ecoTEC with the actoSTOR module — rather than the plain combi's sensor. Second, the ecoTEC pro table does not carry F.12 at all, so a pro showing it would be unusual.
The 2022 CS/1-5 prints F.012 as "the domestic hot water connection temperature sensor has short-circuited" with the wiring harness and the sensor as causes, and Vaillant's UK fault-code page lists F.12 without text. The check list is the NTC plug, cable harness, sensor and PCB, all inside the casing.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Outlet sensor failed short | The NTC on the hot water outlet has broken down and reads near zero resistance. The board rejects it and stops hot water production rather than run to a phantom temperature. | Common |
| Short circuit in the harness | Vaillant name the cable harness on every version of the row. The lead runs through a crowded part of a combi and can chafe against the plate heat exchanger or the pipework. | Common |
| Water on the plug | The outlet sensor sits on wet pipework; a weep from the secondary heat exchanger connections onto the plug bridges the pins and reads as a short until it dries. | Occasional |
| PCB input fault | Last on Vaillant's list and last in practice. Suspected only when a good sensor on a proven harness still reads short. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£180
Time on site
45 minutes
Outlet sensor £110 to £180 fitted. A harness or plug repair usually falls within a call-out of £80 to £120.
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
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.