Vaillant F85 fault code: the temperature sensors are fitted in the wrong place

What does the Vaillant F85 fault code mean?

Vaillant F85 means the boiler's two water temperature sensors, the flow sensor and the return sensor, have been fitted in the wrong place, on the same pipe or on the wrong pipe. The readings stop making sense, so the boiler locks out. There is no home fix: it needs a Gas Safe registered engineer to move or replace the sensors.

Vaillant words this code three different ways depending on the model and the manual. The ecoTEC pro installation and maintenance instructions, document 0020209589_05 dated 17 February 2021, call it “F.85 Fault: NTCs fitted incorrectly”. The ecoTEC plus installation and maintenance instructions, document 0020209591_00, call it “F.85 Fault: Flow and return temperature sensors incorrectly installed”. The ecoTEC plus open vent manual, document 0020238195_08 dated 30 March 2021, shortens it to “F.85 Fault: Temperature sensor”.

The label changes, but the cause column does not. All of those manuals give the same reason: “The flow and/or return temperature sensors have been installed on the same pipe/incorrect pipe”. The remedy column is equally short: “Check: Flow/return temperature sensor (correct thermal connection).” In plain terms, the control board is being told two temperatures that cannot both be true, so it stops the boiler rather than fire the burner without knowing how hot the water really is.

One thing worth knowing before you go any further. F.85 is not on Vaillant's public fault-code page at vaillant.co.uk/service/boiler-fault-codes/. We checked that page on 14 August 2026 and listed every code on it. F.83, F.84 and F.88 are all there. F.85 is not. That page does tell you to contact Vaillant on 0344 736 0049 for a code it does not list, but it gives no entry for F.85 itself. So if you looked your code up on Vaillant's own website and came away with nothing, that is why. The code is real and it is documented, just in the installation and servicing manuals that go to installers rather than on the consumer page.

Key facts

The short version, before the detail.

• Meaning: the flow and return temperature sensors have been fitted on the same pipe or the wrong pipe, so the boiler cannot trust the readings and locks out.
• Can I fix it myself? No. Every cause Vaillant lists sits behind the boiler casing. Your only safe steps are a thermostat check, a pressure reading, a look at the outside of the boiler, and one reset.
• It is an engineer job either way. If one reset clears F85, that has not fixed it: a sensor that clears and then comes back is usually one that is not clipped down properly, so still book a Gas Safe registered engineer.
• Most of the causes Vaillant lists follow work on the boiler: a new install, a swapped heat exchanger, a service, a part change or a board replacement. Tell the engineer exactly what was done and when.
• Check whether the repair should cost you anything before you book. If the boiler is still under its manufacturer guarantee, or was installed or serviced in the last few weeks, ring that installer or Vaillant on 0344 736 0049 first.
• Models: confirmed in official Vaillant UK and Ireland manuals for the ecoTEC pro (VUW, document 0020209589_05, dated 17 February 2021), the ecoTEC plus (VU and VUW, document 0020209591_00) and the ecoTEC plus open vent (VU ..6/6-5 OVZ, document 0020238195_08, dated 30 March 2021).
• Not every Vaillant uses it. The current ecoTEC plus VU GB .../55 manual, document 0020134823_10 dated 27 February 2025, has no F.85 at all; its fault table lists F.83 and F.84 and then moves on. F.85 is not on Vaillant's consumer fault-code page either, checked 14 August 2026.
• Smart Plan boiler and central heating cover is a service plan, not insurance: parts and labour up to £500 in a 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7, for the modules on your cover sheet only. You pay anything above the limit.
• Cover is for future breakdowns. A fault the boiler is already showing when you join is treated in the terms as a defect that existed before you started, so it is quoted for separately rather than covered. For an F85 on the display today, book a one-off repair.
• One-off repair visits run Monday to Friday, 09:00 to 17:00, and carry no workmanship guarantee. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown.
• 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 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 Vaillant boiler shown F85?

Most of the causes Vaillant lists for F85 involve somebody having had the casing off. That is not a criticism of your installer: the sensors clip onto the pipes and have to be on the right one with good thermal contact, and Vaillant's own cause line is that they end up on the same pipe or the wrong pipe. But it does change what you should say when you ring for help. If the boiler was fine last week and somebody worked on it since, say so first. It is the single most useful sentence you can give the engineer.

The other pattern worth spotting is timing. A sensor on the wrong pipe usually shows up straight away, within hours or days of the work. A sensor that is on the right pipe but not clipped down properly tends to be intermittent, clearing on a reset and coming back later. A sensor that has actually failed tends to be what is left once an engineer has confirmed both are on the correct pipes and properly clipped, and the code still returns.

The table below runs through the causes Vaillant lists, what tells them apart, and who can safely deal with each one.

Which cause is mine, and who fixes it?

Can I fix a Vaillant F85 myself?

No, and it would be unfair to pretend otherwise. Every cause Vaillant lists for F85 is behind the casing, and the casing is not yours to open. What you can usefully do is rule out the simple things and gather the two or three facts that will make the engineer's visit shorter. That is the whole list, and it is short on purpose.

1. Check the room thermostat is switched on, is set above the current room temperature, and has working batteries. It will not clear F85, but it rules the controls out before anyone visits.

2. Read the pressure gauge on the front of the boiler and write the number down. The engineer will ask for it, and the neighbouring F.83 code is usually caused by too little water in the boiler or a sensor in the wrong position. Write the reading down rather than topping the system up yourself.

3. Look at the outside of the boiler and the pipes around it for obvious damage or signs of a water leak. Do not remove the casing or any cover to do this.

4. Press the reset button once, exactly as your Vaillant user guide describes, and wait to see whether the boiler restarts. Reset only once. If F85 comes back, stop and book a Gas Safe registered engineer rather than resetting again.

5. Write down what work has been done on the boiler recently, what it was, and roughly when. An install, a service, a heat exchanger swap or any part replacement all count.

6. If the boiler was installed or serviced in the last few weeks, or is still under its manufacturer guarantee, ring that installer or Vaillant on 0344 736 0049 before you pay anyone for a repair.

Stop there. Checking which pipe each sensor is clipped to, swapping inverted sensors back, measuring the resistance across a thermistor, testing the wiring harness or reseating plugs on the circuit board are all engineer jobs, because they mean an open casing and live electrical testing.

Repeated resetting is the one thing genuinely worth avoiding. F85 is not a glitch that clears itself. If the sensors are on the wrong pipes, they will still be on the wrong pipes after the tenth reset, and all you have done is delay the repair.

Is it F84 or F85, and does the difference matter?

F.84 is the code people most often mix up with F.85, and it is easy to see why. They sit next to each other, they use the same two sensors, and their causes overlap. Vaillant's fault tables word F.84 as “Fault: NTC temp. diff. implausible” or “Fault: Flow/return temperature sensor temperature difference implausible”: the difference between the two readings does not make sense, often because the sensors have been inverted. F.85 is the more specific complaint, that the sensors are installed on the same pipe or the wrong pipe in the first place. If you are reading a small display in a dark cupboard, it is worth a second look at which number you actually have.

F.83 is the other near neighbour. Vaillant words it as “Fault: NTC temp. gradient”, “Fault: Dry fire” or “Fault: Flow and/or return temperature sensor temperature change”, depending on the manual. It means that when the burner starts, the temperature change at the flow or return sensor is missing or too small, usually because there is too little water in the boiler or a sensor is sitting in the wrong position on the pipe.

Above F85, the codes move away from the temperature sensors entirely. In the ecoTEC plus open vent manual, document 0020238195_08, F.86 is “Fault: Underfloor heating contact”, where the underfloor heating contact is open or its sensor is disconnected or defective, and F.87 is “Fault: Electrodes”, where the electrodes are not connected, are connected incorrectly, or there is a short circuit in the cable harness. F.88 is “Fault: Gas valve” or “Fault: Gas valve assembly”, again from a missing or incorrect connection or a short in the harness. None of those three is a temperature-sensor fault, so if your display shows one of them, this page is not the one you want.

When should I call a Gas Safe registered engineer?

As soon as F85 returns after one reset. There is no waiting-it-out version of this fault, because nothing about a misplaced sensor changes on its own. If the boiler is your only source of heating and hot water and the weather is cold, say that plainly when you book. It will not change the published call-out hours below, but it does tell whoever takes the call how your job sits against everything else that day.

Tell whoever attends three things: the code, the pressure reading you wrote down, and what work was done on the boiler recently. Most of the causes Vaillant lists follow recent work, so that last detail can be the difference between a diagnosis in ten minutes and an hour of hunting. If the boiler was installed or serviced in the last few weeks, or is still under its manufacturer guarantee, ring that installer or Vaillant on 0344 736 0049 first, because they know exactly what they changed and the work may already be looked after.

You can book a one-off repair with us, or call us on 0333 772 6247. 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. One-off repair visits run Monday to Friday, 09:00 to 17:00, and they carry no workmanship guarantee. Cover-plan call-outs are a different product and run Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown; a fault reported outside those hours is normally booked for the next working day. The two-working-day figure is a target for an engineer attending, not a promise that the repair finishes in that time, because parts can mean a second visit. A one-off repair is a paid job: the engineer diagnoses the fault, then we quote you for any parts and extra labour before that work goes ahead.

If you would rather have future repairs looked after, a Smart Plan boiler and central heating module covers parts and labour up to £500 in a 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and you pay anything above that limit yourself. One thing to be straight about: cover is for future breakdowns. The F85 on your display today is a fault the boiler already has, and the terms treat a fault in existence before you join as a defect, which we quote for separately rather than cover. So a new plan will not pay for tonight's F85, and a one-off repair is the route for that. Cover is modular, so you pick only the parts of your home you want covered. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits 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 your limits rather than paying out a cash sum. It comes from UK Boiler Company Ltd (Companies House 09078640), trading since 2014.

One last thing. Every cause Vaillant lists for F85 is behind the casing, and the boiler has locked itself out precisely so that it does not run without a water temperature it can trust. Leave the casing to the engineer. And if you ever smell gas or suspect carbon monoxide, do not wait for an engineer. Call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F85 fault code FAQs

Seeing F85? Book a Gas Safe registered engineer.

Book a one-off repair and a Gas Safe registered engineer from our national network can check where your flow and return sensors are fitted. One-off repair visits run Monday to Friday, 09:00 to 17:00, and carry no workmanship guarantee. It is a paid job: the engineer diagnoses the fault, then you get a quote for any parts and extra labour before that work goes ahead. Call 0333 772 6247. Before you book, if the boiler was installed or serviced recently, or is still under its manufacturer guarantee, ring that installer or Vaillant on 0344 736 0049 first, because the repair may already be looked after. A Smart Plan boiler and central heating module is a separate thing, for future breakdowns rather than the F85 showing today: a fault the boiler already has when you join is quoted for separately rather than covered. The module covers parts and labour up to £500 in a 12-month period for a boiler under 7 years old, or up to £200 if it is older, and you pay anything above that. It is a service plan, not insurance. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests. Cover-plan call-outs are Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown. Using a module starts a 12-month agreement period once you first 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.

Helpful links

• All Vaillant fault codes
• Vaillant F83 fault code (the neighbouring temperature-sensor code)
• Vaillant boiler help and guides
• Book a one-off boiler repair
• Boiler and central heating cover
• Build your cover, boiler module
• Vaillant official fault-code list (F85 is not on it)
• Check an engineer on the Gas Safe Register