Vaillant F10 fault code: the flow temperature sensor has short-circuited

What does Vaillant F10 mean?

Vaillant F10 means the flow temperature sensor has short-circuited, so the boiler no longer knows how hot the water leaving it is. It locks out on purpose rather than fire the burner blind. You cannot fix this yourself. Reset once, and if F.10 comes back, book a Gas Safe registered engineer.

Vaillant publishes this one for homeowners. On its own consumer fault-code page at vaillant.co.uk the entry for F.10 reads: "The flow temperature sensor has short-circuited." The trade documents say the same thing in trade language: the ecoTEC plus installation and maintenance instructions list it as "Flow NTC short circuit", with the causes given as "NTC defective, short circuit in cable harness, cable/casing".

The sensor Vaillant calls the flow NTC is a thermistor that reads the temperature of the water leaving the heat exchanger. When that sensor or its wiring short-circuits, the reading becomes meaningless, so the boiler shuts down. It is a sensor or wiring fault. It is not a fault with the water, the gas or the pressure, so topping up the pressure will not clear it.

One thing to check before you read on: look at the badge on the front of your boiler. Older Vaillant manuals print this code as "F10" and current ones print it as "F.10", and both mean the same fault. But Ideal, Baxi and Worcester also use short codes without a decimal point, and an Ideal "F1" means something completely different. If your boiler is not a Vaillant, this page is about the wrong appliance.

Key facts

The short version, if you only read one thing. The fault first, then our terms if you want us to do the repair.

• Meaning: the flow temperature sensor has short-circuited, which is Vaillant's own consumer-page wording. Installers know it as a flow NTC short circuit.
• DIY-fixable? No. The only safe step is a single reset following your Vaillant user guide, plus a few checks you can make without touching the boiler.
• Engineer job as soon as F.10 returns after that one reset, because testing or replacing the sensor and its wiring means opening the casing, and Vaillant says opening the casing can be dangerous.
• It is on Vaillant's public list, unlike some codes. You can look it up on vaillant.co.uk yourself, spelled exactly "F.10", and it is also in the installer manuals.
• Models: listed in the official instructions for ecoTEC plus (612, 615, 618, 624, 630, 637, 824, 831, 837), ecoFIT pure (412, 415, 418, 425, 430, 435, and VU 126/6-3 OV to VU 356/6-3 OV H-GB), ecoFIT sustain, and the ecoTEC plus 80, 100 and 120kW commercial range. Vaillant also lists F.10 on its general consumer page rather than per model, so check your own manual if yours is not here.
• No heating tonight: keep one room warm with a heater you already own and trust, close the internal doors, and do not keep resetting the boiler.
• One-off repair: visits run Monday to Friday, 09:00 to 17:00, with a £95 call-out fee, paid in advance, covering the visit and the diagnosis. Any parts and further labour are quoted separately, and one-off repairs carry no workmanship guarantee.
• Cover is for future breakdowns, not a fault your boiler is already showing, so a plan bought tonight would not pay for tonight's F10. For the next fault, Smart Plan boiler and central heating cover pays parts and labour up to £500 in any 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. It is a service plan, not insurance, and the limits apply to the modules you have chosen.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown as set out in the plan terms. A £95 call-out fee, paid in advance, also applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why has my boiler thrown an F10?

Because something in the flow temperature sensor circuit has shorted. Vaillant lists two possible causes on its consumer page: "Fault in the electrical connection for the flow temperature sensor" and "Flow temperature sensor defective". The installer manuals add the detail that installers actually see, naming a defective NTC, a short circuit in the cable harness, or the cable shorting against the casing.

The pattern matters more than the code itself. A sensor that has failed outright usually locks the boiler out and keeps it there. A chafed loom often comes and goes. A fault that appeared the day after a service points somewhere different again. Telling the engineer which of these you are seeing saves time on the visit.

Here are the causes Vaillant and its manuals name, what each one tends to look like from the outside, and who can safely deal with it.

F10 causes and who fixes them

Can I fix a Vaillant F10 myself?

No, and it is worth being straight about that rather than sending you round the houses. The sensor and its wiring are inside the casing, and Vaillant's own consumer page says: "This is a case for a Vaillant engineer. Don't try to solve it yourself as opening the boiler casing can be dangerous."

There is still a short list of things you can safely do. The first one might get your heating back tonight. The rest will not fix anything, but they answer the questions the engineer would otherwise ask on the doorstep, so the visit starts with the diagnosis rather than the interview.

1. Reset the boiler once, following the reset instructions in your own Vaillant user guide. Vaillant says resetting can sometimes clear a fault or get the boiler going temporarily. If F10 comes back, stop there and book an engineer.

2. Check your room thermostat is calling for heat and its batteries are good, so you can tell the engineer whether the boiler is being asked to fire at all.

3. Read the pressure gauge on the front of the boiler and note the figure down. F10 is not a pressure fault, but the reading is useful information for the engineer.

4. Look at the outside of the boiler for anything obviously wrong, such as water dripping from underneath or staining on the casing. Do not open, unclip or remove the casing.

5. Check whether your other gas appliances and your electricity are working normally, so a wider supply problem can be ruled out.

Stop at that point. Resistance-testing the flow NTC against Vaillant's published values, replacing the sensor, repairing the loom, reseating the multiple plug on the printed circuit board, testing the board, and tracing an internal leak are all engineer work, every one of them behind the casing.

Do not sit there resetting it repeatedly to force the boiler past the lockout. A reset clears the lockout, it does not repair a shorted circuit, and the lockout is the boiler protecting itself.

If you have no heating and it is cold, keep one room warm with an electric heater you already own and trust, close the internal doors, and run hot taps as little as possible until the engineer has been. If someone in the house has reached state pension age, is pregnant, has young children, or lives with a disability or a long-term medical condition, contact your energy supplier or network operator and ask to join their Priority Services Register, and check whether your local council can help with emergency heating.

Is it F10, F.00, or an Ideal F1?

F.10 and F.00 are the same sensor failing in opposite directions, which is why people mix them up. Vaillant describes F.10 as "The flow temperature sensor has short-circuited" and the interrupted version as "The flow temperature sensor has been interrupted". Interrupted means an open circuit, a break in the circuit. Short-circuited means the circuit has bridged. Both lock the boiler out and both need an engineer.

Watch the spelling if you go looking. Your display and the installer manuals write that interrupted fault as F.00, but Vaillant's public fault-code page writes the same fault one digit shorter, as "F.0". Checked on 14 August 2026, that page carries no leading-zero codes at all. F.10 itself is spelled the same in both places, so looking that one up is straightforward.

The return pipe has its own matching pair. The interrupted version is F.01 on your display and on the manuals, listed as "F.1 The return temperature sensor has been interrupted" on Vaillant's public page. The short-circuit version is F.11, "The return temperature sensor has short-circuited", which makes F.10 and F.11 the identical fault type, one on the flow sensor and one on the return sensor. F.12 is a short circuit on the domestic hot water connection temperature sensor, which is only relevant on system boilers and cylinder setups. F.13 is a cylinder temperature sensor short circuit.

F.20 turns up in the same conversation. Vaillant describes it as "The safety thermal cut out has operated shutting the boiler off", and it can follow a sensor fault, so it sometimes appears alongside F.10.

The one to be careful about is not a Vaillant code at all. Ideal writes its codes with no decimal point, and an Ideal F1 on most Logic and Vogue boilers means low water pressure, which is a completely different fault with a completely different fix. Vaillant F.10 is a short-circuited sensor and is engineer-only. So check the badge on your boiler before you act on either, and if it turns out to be an Ideal, use Ideal's own published fault-code guidance rather than this page.

When should I call a Gas Safe registered engineer?

As soon as F10 comes back after that single reset. There is no second thing to try. The engineer will typically resistance-test the flow NTC against Vaillant's published values, check the cable harness and the multiple plug on the circuit board, and replace whatever has shorted. If the code is alternating with F.00, F.01 or F.11, expect the NTC sensors to be looked at and changed as a set, because that is how a wider sensor-circuit fault gets narrowed down.

If the code appeared within days of a service or a repair, call the engineer who did that work first. A plug refitted wrongly or a cable pinched against the casing is a known cause, and it belongs to the job it came from.

You can book a one-off repair with us, or call us on 0333 772 6247. One-off repair visits run Monday to Friday, 09:00 to 17:00, and we book the first engineer slot we have. A £95 call-out fee, paid in advance, covers the engineer coming out and diagnosing the fault. Any parts and further labour are quoted separately and explained to you before the work goes ahead. One-off repairs do not carry a workmanship guarantee, which is one of the real differences between a one-off job and a repair carried out under a cover plan.

We work with a national network of thousands of Gas Safe registered engineers, and we aim to get an engineer out to you within two working days. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown. What counts as an emergency breakdown is set out in the plan terms, so if you have no heating at all and there are young children, elderly or unwell people in the house, say so when you call and ask directly whether it qualifies. Otherwise a fault reported on a Friday evening may wait until Monday.

If you would rather not meet a repair bill cold next time, a Smart Plan boiler and central heating module is worth a look, with one thing said straight first. Cover is for future breakdowns, not a fault your boiler is already showing. An F10 on the display today counts as a pre-existing fault and would not be covered, so book a one-off repair for that one. For the next fault, the module covers parts and labour up to £500 in any 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and anything above that limit you would pay yourself. Cover is modular, so you pick only what you want, and the limits apply to the modules you have chosen.

The rest of the cover terms, so you have them before you decide. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests. Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Smart Plan is a service plan, not insurance. It arranges and funds covered repairs up to the limits on your plan rather than paying out a cash sum, and cover is limited to the modules you have chosen. It is provided by UK Boiler Company Ltd, company number 09078640, trading since 2014.

One safety note. F10 is a sensor fault, not a gas one, but if you ever smell gas or suspect carbon monoxide, do not wait for an engineer. Leave the property and call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F10 fault code FAQs

Seeing F10? Book a Gas Safe registered engineer.

Book a one-off repair and a Gas Safe registered engineer can test the flow temperature sensor and its wiring. One-off repair visits run Monday to Friday, 09:00 to 17:00. A £95 call-out fee, paid in advance, covers the visit and the diagnosis, any parts and further labour are quoted separately before work goes ahead, and one-off repairs carry no workmanship guarantee. Call 0333 772 6247. Cover is a separate thing, and it is for future breakdowns rather than a fault your boiler is already showing, so a plan bought tonight would not pay for tonight's F10. Before you sign up, the terms that matter. Parts and labour up to £500 in any 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. A £95 call-out fee, paid in advance, in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests. A 14-day cooling-off period that ends the moment a service is carried out. A 12-month agreement period from the first time you use a service, with early exit costing the remaining months or 75% of the outstanding balance. Cover is modular, so you pick only what you want, and the limits apply to the modules you have chosen. It is a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. We work with a national network of thousands of Gas Safe registered engineers, and we aim to get an engineer out to you within two working days, with cover-plan call-outs running Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown.

Helpful links

• All Vaillant fault codes
• Vaillant F01 fault code (return temperature sensor)
• Vaillant F20 fault code (safety thermal cut-out)
• Vaillant F71 fault code
• Vaillant F72 fault code
• Vaillant boiler help and guides
• Vaillant fault-code help hub
• Boiler repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register