Vaillant F67 fault code: what the flame signal fault means

What does the Vaillant F67 fault code mean?

Vaillant F67 means the boiler's control board is not getting a believable flame-detection signal, so it locks out on safety grounds. It is not a fix you can do yourself: the flame sensor, its wiring and the main circuit board all sit inside the casing. Reset once, and if F.67 returns, book a Gas Safe registered engineer.

Vaillant's own installation and maintenance instructions for the ecoTEC plus VU, VUW ..6/5-5 combi and system boilers (document 0020244997_07) list it under "Fault: Electronics/flame", with the fault described as "Implausible flame signal" and the instruction "Check: Cable harness, flame monitor, PCB." Older manuals for the ecoTEC plus open vent and the ecoFIT pure combi word the same code differently, as "Value sent back by ASIC is incorrect (flame signal)", but give the identical stated cause: "Implausible flame signal, electronics defective". Both wordings point at the electronics and the flame-detection wiring, not at your gas supply.

In plain terms, the boiler checks what the flame sensor is reporting against what it is actually doing. If the two do not agree, it will not carry on firing on a reading it cannot trust, so it shuts down and shows the code. That is the safety lockout doing its job.

One thing worth knowing before you keep searching. F.67 is not on Vaillant's public fault-code page at vaillant.co.uk/service/boiler-fault-codes/, which lists F.62 as the only code in the whole F.6x range, and it is not in the consumer operating instructions either. It appears only in the installer manuals. If you looked on Vaillant's own website and came away with nothing, you were not doing it wrong.

Key facts

The short version, if you are cold and want the answer now.

• Meaning: the control board is reading an "Implausible flame signal", which is Vaillant's own wording in its installation and maintenance instructions.
• Where Vaillant points the engineer: "Check: Cable harness, flame monitor, PCB." It is an electronics and wiring fault, not a gas supply fault.
• DIY-fixable? No. The only safe steps are a thermostat and pressure check, a look at the outside of the boiler, and one single reset.
• Engineer job if: F.67 comes back after that one reset. Everything on Vaillant's check list sits inside the casing.
• Models we have confirmed it on, from Vaillant's own UK manuals: the ecoTEC plus VU and VUW ..6/5-5 combi and system boilers, the ecoTEC plus VU ..6/6-5 OVZ open vent, the ecoTEC plus 80, 100 and 120 kW, and the ecoFIT pure 825 to 835 combi. Other Vaillant boilers of the same generation may use it too, but those are the four manuals we checked.
• Not on Vaillant's public list: F.67 is absent from vaillant.co.uk/service/boiler-fault-codes/, which carries F.62 as the only F.6x code.
• If you are weighing up a Smart Plan cover plan: cover is for future breakdowns, not for a fault already on your display. In the first 30 days of a new agreement, a call-out costs the £95 call-out fee in advance plus payment for the works in advance. For an F.67 showing now, a one-off repair is the honest route.
• Two different sets of hours, so it is worth separating them: one-off repair appointments run Monday to Friday, 09:00 to 17:00. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00.

Why does my Vaillant show F67?

F.67 appears when the flame signal reaching the control board does not match what the boiler is doing. Vaillant names two causes itself: an implausible flame signal, and defective electronics. Its instruction to the engineer is "Check: Cable harness, flame monitor, PCB."

Vaillant's check list and stated causes point at the flame-detection electrode or its lead, the wiring between the flame monitor and the board, or the board itself. Two further checks are worth mentioning honestly for what they are. Vaillant lists moisture on the electronics under the related flame-detection code F.27, and incorrect earthing under F.28 and F.29. Neither is Vaillant's stated cause for F.67, but both are reasonable follow-on checks for an engineer chasing a flame-signal fault that will not settle.

The table below runs through what an engineer will be looking at, what you might notice at your end, and who can safely deal with each one.

F67 causes and who fixes them

Can I fix a Vaillant F67 myself?

No, and it is worth being straight about that. Every item on Vaillant's own F.67 check list, the cable harness, the flame monitor and the PCB, lives inside the boiler casing. Opening that casing is Gas Safe registered work. What follows is not a repair, it is the short list of things that are genuinely safe to do and that will make the engineer's visit quicker.

1. Read the fault code off the display and write it down exactly as shown, including the dot: F.67.

2. Check the room thermostat and any programmer are calling for heat, so you know the boiler is genuinely locked out rather than simply satisfied.

3. Read the pressure gauge and note the figure. F.67 is not a pressure fault, but the reading is useful information for the engineer.

4. Check whether your other gas appliances still work, a gas hob for example, and whether the rest of the house has power. This is only so the engineer knows your gas and power supply are normal before they arrive. F.67 itself is not a gas supply fault.

5. Look at the outside of the boiler and the flue terminal for obvious damage, staining or water dripping. Do not open the casing or remove any panels.

6. Press the reset button. Vaillant's manual says that if the boiler does not resume normal operation after this, you should call your installation or servicing company, or Vaillant service. Our own advice is to press it once and no more.

7. If F.67 comes back, stop there and book a Gas Safe registered engineer. Vaillant's operating instructions say that if an F.xx fault message occurs, you should contact a competent person.

Reset only once. Repeatedly resetting a boiler that is locking out does not fix anything, it just clears the display on a fault that is still there.

If you ever smell gas while you are checking, stop what you are doing and call the National Gas Emergency Service on 0800 111 999.

Everything beyond that list is engineer work: inspecting, cleaning or replacing the flame-detection electrode and its lead, testing continuity of the cable harness, reseating the multi-plugs on the board, measuring the ionisation current, replacing the PCB and setting its Device Specific Number so the board matches the appliance, checking earthing and bonding, and anything at all involving the gas valve, gas pressures or combustion analysis.

Is my code F67, F68 or F27? How do I tell them apart?

These three are the ones people mix up, because all of them involve the flame. F.67 is "Implausible flame signal", and Vaillant sends the engineer to the cable harness, flame monitor and PCB. F.68 is "Unstable flame (analogue input)", and its listed causes are air in the gas, gas flow pressure too low, an incorrect air ratio, an incorrect gas injector, or an interrupted ionisation lead. F.27 is a safety shutdown for a "Fault in flame detection", listing moisture on the electronics, a defective flame monitor, or a leaking gas solenoid valve. Put simply, F.67 points at the electronics, F.68 points at the gas and combustion side, and F.27 is a safety shutdown on flame detection.

The rest of the F.6x range is worth knowing too, because a misread digit sends an engineer to the wrong part of the boiler. F.61 is a gas valve assembly control fault. F.62 is a gas valve switch-off control fault, and it is the only code in the F.6x range that Vaillant publishes on its public consumer page, where it is worded "The flame switch-off was detected after a delay." F.63 is an EEPROM fault. F.64 covers the electronics, sensor and analogue-to-digital converter. F.65 means the electronics temperature is too high.

No F.66 appears in any of the four Vaillant UK installer manuals we checked, whose fault tables run from F.65 straight to F.67 with nothing in between. If you think you are seeing F.66, take a second look at the display before you quote the code to an engineer: it is most likely F.65, F.67 or F.68.

One more thing that tells you something about your boiler. The newest 2024 ecoTEC plus platform manual uses a renumbered scheme running F.00 to F.36 plus F.70 and F.90, with no F.67 at all. So an F.67 on your display means you have the earlier generation of appliance, not the current platform.

When should I call a Gas Safe registered engineer?

Call one as soon as F.67 comes back after your single reset. There is nothing further you can safely try, and Vaillant's own instruction after that reset is to call your installation or servicing company, or Vaillant service. Booking sooner rather than later also means you are not sitting in the cold waiting for an appointment slot.

What the engineer will actually do is work down Vaillant's check list: cable harness, flame monitor, PCB. In practice that means testing the wiring between the flame monitor and the main board, reseating the plugs on the board, and inspecting the flame-detection electrode and its lead that feed that signal. They can also measure the ionisation current to see what the flame signal is really doing. Only if all of that is sound does the board itself come into question. If a PCB is fitted, they also have to set the Device Specific Number so the new board matches your appliance. Which of the three it turns out to be is a diagnosis the engineer makes on site.

You can book a one-off repair with us and we will arrange for a Gas Safe registered engineer to attend, or call us on 0333 772 6247. We work with a national network of thousands of Gas Safe registered engineers. One-off repair appointments run Monday to Friday, 09:00 to 17:00, and we confirm the appointment with you by phone before an engineer is sent out. If the diagnosis shows that parts or extra labour are needed, we give you a quote for that work and you authorise it before it goes ahead. A one-off repair is a single visit and does not carry a workmanship guarantee.

If you would rather have future breakdowns handled, a Smart Plan boiler and central heating module covers parts and labour up to £500 in any 12-month period of your agreement if your boiler is under 7 years old, or up to £200 if it is over 7. Anything above your cover limit you would pay yourself. Cover is modular, so you pick only what you want: the terms apply only to the products listed on your cover sheet.

Two things matter a lot if your boiler is showing F.67 right now. Cover is for future breakdowns, not for a fault already on the display. In the first 30 days of a new agreement, any call-out you request costs the £95 call-out fee in advance, plus payment for any additional works in advance of the work being completed. The terms also define a defect as a fault deemed to be in existence before your agreement started, and works to repair a defect we have identified are listed as an exclusion. So for a fault happening tonight, a one-off repair is the honest route, and cover is the thing you set up for next time.

One more thing worth knowing before you count on cover for this particular fault. The cover-plan terms say that failure of the boiler heat exchanger or the PCB means the equipment is declared beyond economic repair. Where that happens you are given a contribution payment rather than a repair, set at whichever is lower: the retail value of the equipment including installation, or your cover limit less the value of work already completed. A faulty PCB is one of the three things F.67 points at, so that is a real possibility on this code rather than a remote one.

A few more things worth knowing before you set anything up. The £95 call-out fee, paid in advance, also applies for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days, which is a target rather than a promise. Outside those hours only an emergency breakdown is seen sooner, and the cover-plan terms do not define what counts as one, so a Friday-evening fault can wait until Monday. Using any 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: the terms state in as many words that it is not a contract of insurance. It comes from UK Boiler Company Ltd, which has traded since 2014.

One safety note. F.67 is a flame-signal electronics fault, not a reported gas leak. But 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.

While you are waiting for the repair, never use a gas hob, a gas oven or any unflued heater to warm a room. They are not made for space heating and they can put carbon monoxide into the room. Use only electric heaters designed for room heating, and follow the maker's instructions. If anyone in the home is elderly, disabled, seriously ill or has a young baby, it is worth registering with your energy supplier's Priority Services Register.

Vaillant F67 fault code FAQs

Seeing F67? Let's get an engineer to you.

Book a one-off repair and a Gas Safe registered engineer can work through the flame-detection electrode, the cable harness and the board, or call us on 0333 772 6247. We work with a national network of thousands of Gas Safe registered engineers. One-off repair appointments run Monday to Friday, 09:00 to 17:00, we confirm the appointment with you by phone first, and if parts or extra labour turn out to be needed we quote for that work and you authorise it before it goes ahead. A one-off repair does not carry a workmanship guarantee. If you would rather have future breakdowns handled, a Smart Plan boiler and central heating module covers parts and labour up to £500 in any 12-month period of your agreement for a boiler under 7 years old, or up to £200 if it is over 7, with anything above that limit paid by you. It will not pay for the F.67 you are looking at now: in the first 30 days of a new agreement a call-out costs the £95 call-out fee in advance plus payment for the works in advance, and the terms exclude work on a fault deemed to have existed before your agreement started. That same £95 call-out fee also applies for faults the plan does not cover, no-access visits and early annual-service requests. Cover-plan call-outs are a separate set of hours, Monday to Friday, 08:00 to 18:00 unless it is an emergency breakdown, and we aim to get an engineer out to you within two working days as a target rather than a promise. Using a module starts a 12-month agreement period once you first use a service, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum.

Helpful links

• Build your cover plan
• Boiler repair
• Boiler & central heating cover
• Cover plan terms and conditions
• One-off works terms and conditions
• Vaillant boiler help and guides
• All Vaillant fault codes
• Vaillant F65 fault code (the code before this one)
• Vaillant F62 fault code (the only F.6x code Vaillant publishes)
• Vaillant F27 fault code (the other flame-detection code)
• Vaillant F64 fault code
• Vaillant fault-code help hub
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register