Vaillant F.91 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.91 fault code mean?
The F.91 fault code on a Vaillant boiler has two distinct meanings depending on your model, so identifying your boiler before troubleshooting is essential. On most standard Vaillant ecoTEC models, F.91 indicates a problem with the heating return NTC thermistor — either a short circuit in the sensor itself, or a fault in the wiring connecting that sensor to the main PCB. The return NTC is a resistive temperature sensor (it should read approximately 12kΩ at 20°C) that tells the boiler how hot the returning water is; when it reports an implausible value due to a short or open circuit, the boiler locks out for safety. On the Vaillant ecoTEC Plus 937 and 938 (actoSTOR models only), F.91 almost always appears alongside F.81 and means the integrated domestic hot water storage tanks have failed to reach the target temperature within the five-minute heating window. This is a separate diagnostic path related to the actoSTOR hot water system rather than the main heating sensor.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Short circuit in the heating return NTC thermistor (standard models) Common
The most frequent cause on standard ecoTEC boilers. The return NTC thermistor develops an internal short circuit, causing it to report an out-of-range resistance value. The boiler's PCB detects this as an implausible temperature reading and triggers F.91 to prevent unsafe operation.
- Wiring harness fault between return NTC and PCB (standard models) Common
Corrosion, chafing, or a loose plug connector anywhere along the cable run between the return temperature sensor and the main circuit board can cause the same short-circuit or open-circuit signal as a failed sensor. An engineer will test continuity and resistance through the harness to isolate the fault.
- Failed actoSTOR domestic hot water pump (937/938 models only) Common
On the ecoTEC Plus 937 and 938, the secondary circulation pump that moves water through the actoSTOR hot water tanks is the most common culprit when F.81 and F.91 appear together. These pumps tend to fail on boilers aged roughly 10–12 years. Without a working pump, the tanks cannot be heated within the five-minute window and the boiler flags F.91.
- Blocked plate heat exchanger (937/938 models only) Sometimes
Limescale or debris build-up in the plate-to-plate heat exchanger on actoSTOR models restricts heat transfer to the hot water tanks, meaning the target temperature cannot be reached in time. This is more prevalent in hard-water areas.
- Loose or disconnected NTC sensor plug (standard models) Sometimes
Vibration over time can cause the sensor connector to work loose from its housing. This interrupts the circuit in the same way as a wiring fault and is relatively quick for an engineer to check and remedy.
- Faulty cylinder charging or actoSTOR sensor (937/938 models only) Sometimes
A defective cylinder sensor or impeller sensor on 937/938 models can give false readings that prevent the boiler from confirming that the tanks have reached temperature, triggering F.91 even if the pump and heat exchanger are functioning correctly.
- Secondary PCB failure (937/938 models only) Rare
The 937/938 uses a secondary control board to manage the actoSTOR system. If this board develops a fault it can prevent the hot water circuit from operating correctly, producing F.81/F.91. This is a less common but more expensive diagnosis.
- Main PCB fault (standard models) Rare
If the sensor and wiring both test within specification, the main circuit board itself may be misreading the NTC input due to a failed input circuit. PCB faults are uncommon but do occur, particularly on older appliances.
How to fix it
- Identify your boiler model before doing anything else DIY safe
Check the data plate on the front or inside the casing door of your boiler. If it says ecoTEC Plus 937 or 938, your F.91 is almost certainly appearing alongside F.81 and relates to the actoSTOR hot water system. On any other ecoTEC model, F.91 points to the return NTC thermistor or its wiring. Knowing which scenario applies will save time and money when you speak to an engineer.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, fire) are working normally. If there is no gas, contact your gas supplier rather than investigating the boiler further.
- Check your boiler's system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 0.8 bar, top it up via the filling loop according to your boiler's user guide. Low pressure is unlikely to cause F.91 directly, but it is worth ruling out before calling an engineer.
- Attempt a boiler reset DIY safe
On Vaillant boilers, the reset button is on the main display panel — look for the flame icon with a line through it. Press and hold it for approximately three seconds until the display changes and the fault code clears. If the boiler fires up and runs normally, monitor it over the next few hours. If F.91 returns, do not keep resetting repeatedly — a recurring fault needs professional diagnosis, not repeated resets.
- Do not open the boiler casing or attempt sensor or wiring work yourself Gas Safe engineer
Replacing or testing the return NTC thermistor, checking continuity of the wiring harness, or accessing any internal component requires the boiler casing to be removed. This work must only be carried out by a Gas Safe registered engineer. Attempting it yourself is unsafe and may invalidate your warranty or home insurance.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
An engineer will use a multimeter to check the resistance of the return NTC thermistor (expected: approximately 12kΩ at 20°C) and test continuity through the wiring harness. On 937/938 models, they will assess the actoSTOR pump, plate heat exchanger, and cylinder sensors. Depending on findings, they will replace the faulty component — sensor, harness, pump, or in rare cases the PCB. If you have a 937/938, try to find an engineer with specific experience of actoSTOR systems to avoid unnecessary parts being replaced.
Parts you may need
- Heating return NTC thermistor (standard ecoTEC models) · from £25
- Wiring harness / sensor cable loom · from £45
- actoSTOR secondary circulation pump (937/938 models) · from £200
- Plate heat exchanger (937/938 models) · from £120
- Cylinder charging sensor / actoSTOR sensor · from £35
- Main PCB (standard ecoTEC models) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£380, depending on the underlying cause.
Frequently asked questions
Why does F.91 appear alongside F.81 on my Vaillant boiler?
If your boiler is a Vaillant ecoTEC Plus 937 or 938, seeing both F.81 and F.91 together is the normal presentation for a fault in the actoSTOR domestic hot water system — most commonly a failed secondary circulation pump. The two codes appear simultaneously because the boiler could not heat the storage tanks within the five-minute window and reports both a timing failure (F.91) and the associated system fault (F.81). On any other ecoTEC model, F.91 alone relates to the return NTC thermistor, not the hot water system.
How much does it cost to fix a Vaillant F.91 fault?
On standard ecoTEC models, replacing the return NTC thermistor or its wiring typically costs £100–£200 including parts and labour. On Vaillant ecoTEC Plus 937 and 938 models, replacing the actoSTOR pump — the most common repair — usually comes to around £330–£350 all in, as the pump itself costs roughly £200 and access is time-consuming because the cylinder must be drained first. In rare cases where the main PCB or secondary PCB is at fault, costs can rise to £400–£600 or more, though this is uncommon.
Can I reset my Vaillant boiler to clear the F.91 code?
Yes, you can attempt a reset by pressing and holding the reset button (the flame-with-a-line icon on the display panel) for around three seconds. If the fault clears and the boiler runs without issue, keep an eye on it. However, if F.91 comes back after one or two resets, stop resetting and call a Gas Safe registered engineer — the underlying fault will not go away on its own and repeated resets can mask a developing problem.
Does an F.91 fault mean I need a new boiler?
In most cases, no. On standard models, a faulty NTC thermistor or wiring harness is a relatively inexpensive and straightforward repair. Even on the 937/938 with a failed actoSTOR pump, the repair is typically £330–£350 — far less than boiler replacement. The exception would be if diagnosis reveals a failed main PCB on an older boiler, where repair costs may approach the value of the appliance. A Gas Safe engineer can advise on whether repair or replacement makes more financial sense based on the age and condition of your specific boiler.