Vaillant F68 fault code: what an unstable flame signal means

What does the Vaillant F68 fault code mean?

Vaillant F68 means the boiler lit, but the signal that proves a flame is present kept wavering, so it shut the burner down and locked out. It points at the gas supply, the combustion settings, the flue or condensate route, or the flame-sensing electrode. None of that is a homeowner fix. Reset once, and if F68 returns, book a Gas Safe registered engineer.

Vaillant writes the code with a decimal, as F.68. The installation and maintenance instructions for the ecoTEC plus combi and system (Vaillant document 0020244997_07, dated 15 July 2021) list it as "F.68 Fault: Flame signal unstable", with the cause given as "Flame monitor reports an unstable flame signal". That is an installer manual, not the homeowner leaflet.

You will not find F.68 on Vaillant's public fault-code page at vaillant.co.uk/service/boiler-fault-codes/. At the time of writing, checked in August 2026, that page carries 27 codes, from F.0 to F.88, and F.68 is not one of them, so it simply tells you to ring 0344 736 0049. If you searched Vaillant's own site and came away with nothing, that is why. The code is genuine, it just lives in the installer manual.

Vaillant boilers do not look at the flame. They sense it electrically, through an ionisation electrode inside the sealed combustion chamber. If that signal wavers, the boiler cannot treat the flame as safe, so it stops. Locking out is the boiler doing its job, not failing at it.

One thing that trips people up: the wording changes between ranges. The ecoTEC plus manual says "Fault: Flame signal unstable". The ecoFIT pure and ecoFIT sustain manuals say "Fault: Unstable flame signal". The ecoTEC plus open vent and Vaillant HOME manuals say "Fault: Unstable flame (analogue input)". Same fault, three phrasings, which is why search results seem to disagree.

Key facts

The short version, if you are reading this stood in a cold kitchen.

• Meaning: the flame monitor reported an unstable flame signal, so the boiler shut the burner down (Vaillant ecoTEC plus manual, doc 0020244997_07).
• DIY-fixable? No. The safe list is a thermostat check, a pressure reading, a look at the outside pipework, and one reset.
• Engineer job if: F68 comes back after that single reset. Every one of Vaillant's listed checks needs test equipment or access inside the boiler.
• Affects: ecoTEC plus combi, system and open vent, ecoTEC exclusive, ecoFIT pure (412, 415, 418, 425, 430 and 435), ecoFIT sustain (415, 418 and 430) and Vaillant HOME. Most modern Vaillant boilers can show it.
• Not on Vaillant's public list: checked in August 2026, the consumer fault-code page carries 27 codes and F.68 is not among them, so it is documented in the installer manual only.
• Most common trigger in winter: a blocked or frozen condensate route, which Vaillant lists as a direct cause.
• Cover is for future breakdowns. A fault your boiler already has when you join counts as a defect that was there before the plan started, so an F68 on the display today is a job for a one-off repair, not a new plan.
• One-off repair appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays, and are not covered by a workmanship guarantee. The visit covers the diagnosis, and you get a quote before any parts or extra labour go ahead.
• Smart Plan boiler and central heating cover: parts and labour up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is over 7. You pay anything above that limit yourself, and cover applies only to the modules on your cover sheet.
• A £95 call-out fee, paid in advance, applies on a cover plan in your first 30 days, 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, with an engineer attending outside those hours only for a genuine emergency breakdown.
• 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 does my Vaillant keep locking out on F68?

Because something is making the flame unsteady, and Vaillant does not pin F.68 on one part. Its measure for the code reads "Check: Air ratio, gas flow pressure, condensate route (blockage), gas injector, ionisation flow (cable, electrode), flue gas recirculation." That is six separate things to rule out, which is why F.68 usually needs a proper diagnostic visit rather than a guess at a part.

The pattern around the fault often tells you more than the code does. F.68 that arrives on the first hard frost points at the condensate route as the first thing to check. F.68 that started the day after a repair or an LPG conversion points at the wrong injector or restrictor. F.68 that only bites in high wind points at the flue. Worth mentioning all of that when you book.

The ecoFIT and Vaillant HOME manuals add two more possibilities to the same code: "air in gas" and "gas flow pressure too low". Both are about the gas reaching the burner cleanly and steadily.

F68 causes and who fixes them

Can I fix a Vaillant F68 myself?

Before you touch anything: if you can smell gas, if your carbon monoxide alarm is sounding, if the boiler is leaking, or if you can see scorching or soot around it, do not reset the boiler and do not light anything. Call the National Gas Emergency Service on 0800 111 999.

Otherwise, no, you cannot fix F.68 yourself, and it is worth being straight about why. F.68 is a flame-safety fault. Every item on Vaillant's own fix list needs test equipment or access inside the sealed parts of the boiler. There is no filter to clean, no valve to turn, no bleeding to do. What you can usefully do is gather information and try one reset.

Work through these in order.

1. Check the room thermostat is turned up and calling for heat, so you know the boiler is genuinely trying to fire.

2. Read the pressure gauge on the front of the boiler and write the figure down. It is not the cause of F.68, but the engineer will ask for it.

3. Only if you cannot smell gas, check whether your other gas appliances still work, a gas hob for example. If nothing gas-fired lights, the problem may be the gas supply to the whole property rather than the boiler.

4. From outside, look at the flue terminal and the white plastic condensate pipe for obvious ice, damage or an obstruction, and only if you can see them safely from ground level. Do not use a ladder, do not climb, and do not open the boiler casing, remove any panel or touch the flue.

5. Reset the boiler once, following the reset instructions in your Vaillant user guide. If F.68 comes back, stop there and book a Gas Safe registered engineer. Repeated resets on a flame fault are not safe.

Stop at that point. Measuring gas working pressure, running a flue gas analysis, adjusting the air and gas ratio, measuring the ionisation current, inspecting or replacing the flame-sensing electrode, clearing the condensate trap inside the boiler, checking flue integrity and seals, verifying the injector or restrictor, and testing the earthing, harness, flame monitor and PCB are all engineer work. So is removing any casing panel, for any reason.

If your condensate pipe outside is visibly frozen, that is still not a job to improvise with boiling water and a ladder. Tell the engineer what you saw and let them deal with it safely.

You will most likely be without heating, and probably hot water, until the engineer comes, so plan for that. Never use a gas hob or oven to heat a room.

If you smell gas at any point, do not reset anything. Call the National Gas Emergency Service on 0800 111 999 straight away.

Is F68 the same as F67, F29 or F28?

No, and the differences matter, because they send an engineer to different parts of the boiler. F.68 is "Fault: Flame signal unstable": the flame was there, but the proof of it kept wavering, which points at the burner side, the gas, the flue or the electrode.

F.67 is its closest twin and the easiest to mix up. Vaillant words it as "Fault: Electronics/flame", with the cause "Implausible flame signal, electronics defective". Same signal, different suspect: the electronics rather than the burner.

F.29 is "Fault: Flame loss", meaning the flame actually went out during operation and would not relight. F.28 means ignition was unsuccessful at start-up, so the boiler never lit at all. Both get confused with F.68 because all three look like a boiler that will not stay lit.

Two more sit close by on the flame and gas side. F.62 is "Fault: Gas valve assembly switch-off delay", a delayed switch-off of the gas valve or delayed extinguishing of the flame signal, and it is one of the few in this group that does appear on Vaillant's public page. F.61 is "Fault: Fuel valve actuation", a short circuit or short to earth in the gas valve cable harness, or a defective gas valve.

The other neighbours are electronics faults rather than flame faults: F.63 is "Fault: EEPROM", F.64 is "Fault: Electronics/NTC", F.65 is "Fault: Electronics temp.", and F.70 is "Fault: Invalid Device Specific Number", usually seen when a display and PCB were replaced together without resetting the DSN.

When should I call a Gas Safe registered engineer?

As soon as F.68 returns after one reset. There is nothing further to try at home, and a flame fault is not a code to keep clearing and hoping. Assume you are without heating until the visit, and never use a gas hob or oven to warm a room in the meantime.

It helps to have four things ready when you ring: the exact code on the display, your boiler model, the pressure reading you wrote down, and the pattern you have noticed, whether that is cold mornings, windy nights, or a fault that started right after other work was done. On a code with six possible causes, that context genuinely shortens the visit.

The engineer will work through Vaillant's own list: gas working pressure on a manometer, a flue gas analysis, the ionisation current and the flame-sensing electrode, the condensate trap and route, the flue and air intake for recirculation, the injector or restrictor, and finally the earthing, harness, flame monitor and PCB. Expect a diagnostic visit rather than a straight part swap.

Call an engineer immediately, rather than working through anything above, if the boiler is leaking or you can see scorching or soot around it. If you smell gas, or your carbon monoxide alarm sounds, stop: call the National Gas Emergency Service on 0800 111 999, open windows, turn off the gas at the meter, do not touch electrical switches, and leave the house.

If you would rather we sorted it, book a one-off repair or call 0333 772 6247, and we can arrange a visit through a national network of thousands of Gas Safe registered engineers. One-off repair appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays, and are not covered by a workmanship guarantee. The visit covers the diagnosis, and you get a quote before any parts or extra labour go ahead, so it is always worth asking what the charge will be before you agree to a visit, from us or from anyone else.

If you are already on a cover plan, we aim to get an engineer out to you within two working days, and cover-plan call-outs run Monday to Friday, 08:00 to 18:00, with an engineer attending outside those hours only for a genuine emergency breakdown. We cannot say in advance whether a given fault will be treated as one, so plan around the published hours.

Vaillant F68 fault code FAQs

Locked out on F68? We can arrange a Gas Safe registered engineer.

Book a one-off repair and we can arrange a Gas Safe registered engineer to diagnose the fault, or call us on 0333 772 6247. One-off repair appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays, and are not covered by a workmanship guarantee; the visit covers the diagnosis, and you get a quote before any parts or extra labour go ahead. Thinking about a cover plan instead? Take it in this order. Cover is for future breakdowns, so a plan taken out tonight will not pay for the F68 already on your display, and 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 which ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. With that clear, a Smart Plan boiler and central heating module covers parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it is over 7. You pay anything above that limit yourself, and cover applies only to the modules on your cover sheet. Cover is modular, so you pick only what you want. On a cover plan we aim to get an engineer out to you within two working days, and cover-plan call-outs run Monday to Friday, 08:00 to 18:00, with an engineer attending outside those hours only for a genuine emergency breakdown. Smart Plan is a service plan, not insurance: it funds covered repairs up to your limit rather than paying out a cash sum, and it comes from UK Boiler Company Ltd, trading since 2014.

Helpful links

• All Vaillant fault codes
• Vaillant F29 fault code (flame loss)
• Vaillant F62 fault code (gas valve switch-off delay)
• Vaillant F61 fault code (gas valve actuation)
• Vaillant F65 fault code (electronics temperature)
• Vaillant boiler help and guides
• Vaillant fault-code help hub
• Boiler repair
• Boiler & central heating cover
• Build your own cover plan
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register