Vaillant F.91 fault codewhat it means and how to fix it
On an ecoTEC plus or exclusive, a sensor on the actoSTOR module is defective. On an ecoFIT, F.91 is the cylinder temperature sensor short-circuited. Two different sensors under one number.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.91
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What F.91 means
F.91 changes meaning between the two families that print it. On the ecoTEC plus ..6/5-5 and its storage combis it is "fault: actoSTOR sensor — sensor on the actoSTOR is defective; check plug, cable harness, sensor", and the code has a second role there: several other rows are marked "only in conjunction with F.91" — F.2, F.3, F.12, F.13, F.80 and F.81 — meaning those sensor faults are read as actoSTOR faults when F.91 is present alongside them. The module carries its own temperature sensors for charging and stratification, and F.91 is the umbrella for a failure among them.
On the ecoFIT pure and ecoFIT sustain the same number is a plain sensor row, printed as "F.12 and F.91 — short circuit: cylinder temperature sensor — NTC sensor defective, short circuit in the cable harness, cable/housing", so an ecoFIT system boiler showing F.91 has a shorted cylinder probe or its cable, nothing to do with an actoSTOR.
The pro, exclusive, ecoTEC sustain and 2022 CS/1-5 tables do not carry F.91 at all. In both meanings the repair is a sensor or its wiring, inside the module or at the cylinder, and in both the boiler stops charging hot water until it is put right.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| actoSTOR sensor failed (ecoTEC plus) | One of the module's charging or stratification sensors has gone open or short. Vaillant's check list is plug, harness, sensor. | Common |
| Cylinder sensor short-circuited (ecoFIT) | The probe in the cylinder or its cable reads near zero resistance. Vaillant's "NTC sensor defective, short circuit in the cable harness, cable/housing". | Common |
| Plug or harness fault | On both versions of the row. The actoSTOR's sensor leads run between two appliances and the cylinder lead is the longest on a system boiler. | Occasional |
| Companion fault reported alongside | On the plus, F.91 can appear with F.2, F.3, F.12, F.13, F.80 or F.81, which then identify which module sensor or function is affected. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£180
Time on site
1 hour
A sensor is £110 to £180 fitted, at the module or the cylinder. A plug or harness repair sits 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
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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
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.