Vaillant F25 fault code: flue gas temperature too high

What does the Vaillant F25 fault code mean?

Vaillant F25 means the boiler has shut itself down because the gases leaving through the flue got too hot, or because the sensor watching that temperature has lost its connection. It is a safety lockout, not a burner breakdown. You cannot fix it yourself, and Vaillant routes this code straight to an installer. The boiler stays locked out until a Gas Safe registered engineer clears the cause.

Vaillant's own consumer fault-code page puts it in seven words: “The flue gas temperature is too high”. Its installation and maintenance manuals are blunter, listing the code as “Safety switch-off: Flue gas temperature too high” (ecoFIT pure installation and maintenance instructions, document 0020230535_01, page 29). The manuals name a specific electrical cause too: a break in the plug connection for the optional flue gas safety temperature limiter (STB), or a break in the cable harness.

Vaillant spells it F.25 with a full stop; the display and most searches drop it, so F25 and F.25 are the same code. It does appear on Vaillant's public fault-code page at vaillant.co.uk, so if you found it there and came away none the wiser, that is because the entry gives you one line of meaning, three possible causes and a line telling you to contact an installer, and no homeowner steps at all. Its advice is “This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.”

Key facts

The fault first, then what you need to know about cover if you are weighing that up too.

• Meaning: a safety shutdown because flue gas temperature is too high, or because the sensor watching it has lost its connection. Vaillant's words: “The flue gas temperature is too high”.
• DIY-fixable? No. Vaillant gives homeowners no self-help steps for F25 at all and routes it to an installer in the first instance.
• Safe to do yourself: read the pressure gauge (Vaillant's normal range is 1 to 1.5 bar), check the thermostat, look for obvious external damage, and reset once.
• Engineer job: always, whether or not the single reset clears it. A reset clears the lockout, not the cause, so a boiler that fires up again has still not been fixed.
• Models: listed in the official manuals for the ecoFIT pure (document 0020230535_01) and the ecoTEC plus, including the 424 and the 612, 615, 618, 624, 630, 637, 824, 831 and 837 series, and referenced for the ecoTEC pro 24 and 28.
• On Vaillant's own public fault-code page: yes, but the entry gives no homeowner steps, unlike its entry for F28.
• Booking a one-off repair: visits run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, and the engineer tells you what is needed and what it costs before any work goes ahead. One-off repairs do not carry a workmanship guarantee.
• If you are thinking about a Smart Plan instead, read the next four points first.
• Smart Plan boiler and central heating cover (a service plan, not insurance): parts and labour up to £500 per 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. You pay anything above that limit yourself.
• Cover is for future breakdowns, not a fault your boiler is already showing. An F25 on the display today counts as a pre-existing fault, so a new plan would not pay for it, and a one-off repair is the route for this one.
• 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. Your cover itself applies from the start of your agreement: the first 30 days are a call-out fee window, not a gap in cover.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Outside those hours you are only seen sooner for a genuine emergency breakdown as set out in your terms, so a Friday-evening or weekend fault can wait until Monday.
• 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 is my Vaillant showing F25?

F25 points at heat that is not getting away from the boiler properly, or at the sensor that measures it. Vaillant's public page names three possible causes: “Circulation fault, pump, valve, blockage; Wiring harness defective; Flue gas temperature monitor has triggered.” The installation manuals add the electrical detail: a break in the plug connection for the optional flue gas safety temperature limiter (STB), or a break in the cable harness.

In plain terms, Vaillant's documented causes fall into two groups. Either the water side is not carrying heat away, so the gases going up the flue run hotter than they should, or the electrical path to the sensor watching those gases has been interrupted. Both end in the same lockout.

Here are the causes Vaillant documents, with the sign that points at each one.

F25 causes and who fixes them

Can I fix a Vaillant F25 myself?

Honestly, not much, and Vaillant agrees. Its own fault-code page sends F25 to an installer in the first instance and, unlike its entry for F28, gives homeowners no self-help steps for this code at all.

So what follows is not a fix, and it is not a reason to put off the phone call. It is the small set of things you can safely check, so the engineer turns up already knowing something useful.

1. Read the pressure gauge on the front of the boiler and write the reading down. Vaillant says the normal range is 1 to 1.5 bar, so anything below that is worth telling the engineer.

2. Check the room thermostat is set correctly and is calling for heat, so you can say whether the boiler locks out under demand.

3. Look over the outside of the boiler and the visible pipework for obvious damage, leaks or water staining. Do not open the casing.

4. Check whether your other gas appliances, such as the hob or a gas fire, are working normally, and whether the boiler still has power at its fused spur.

5. Reset the boiler once, following the reset instructions in your Vaillant user manual. Reset only once. If F25 comes straight back, stop there.

That is the stop point, and here is why it matters. Everything else on the F25 list sits behind the casing, up the flue or out in the heating system: the flue gas safety temperature limiter and its plug, the cable harness and circuit board connections, the pump, the diverter valve, the internal air vent, the flue route, system valves and sludge. Opening the casing on a gas appliance is not homeowner work.

Repeated resetting is the one thing worth avoiding. A reset clears the lockout, not the cause. If the flue gas temperature is genuinely too high, the safety limiter simply trips again as soon as the boiler fires, so resetting achieves nothing and hides a fault that still needs an engineer. Book one even if the reset appears to hold, because the flue or circulation problem underneath has not gone anywhere.

Is it really F25, or F24, F28 or another code?

Probably F25, if that is what the display shows. But several of its neighbours get mixed up with it, and one of them we cannot confirm at all.

F.24 is the closest. Its manual wording is “Safety switch-off: Temperature rise too fast”, and on Vaillant's consumer page it reads “The temperature increases too quickly”. Both F.24 and F.25 are safety shutdowns, and both can come from poor circulation, which is why they get confused. The difference is what is being measured: F.24 is about how fast the water heats up, F.25 is about how hot the flue gases get. If you have read a page saying F25 means a rise of five degrees a second, that figure belongs to F.24, not F.25.

F.22 and F.23 sit on the water side. F.22 is low water pressure, which Vaillant describes as “The water pressure in your system is below the normal range of 1 to 1.5 bar”. F.23 is too great a spread between the flow and return temperatures, often caused by air, a circulation fault or a defective sensor. Low pressure can also lead to F25, which is why the gauge is worth reading either way.

F.27 and F.28 are flame codes, not heat codes. F.27 is “Safety switch-off: Flame simulation”, a flame signal detected while the burner is off, and it is engineer work, not something to tackle at home. F.28 is “Failure during start-up: ignition unsuccessful”, the far more common no-ignition fault, and it is frequently mistyped as F25 or F20.

F.26 is the odd one out. It is not on Vaillant's public fault-code page and it is not in the ecoFIT pure fault-code table on page 29 of document 0020230535_01. So if you are told your boiler is showing F26, treat it as unconfirmed until an engineer reads the code off the boiler, rather than assuming it sits neatly between F25 and F27.

When should I call a Gas Safe registered engineer?

Straight away, and whether or not that single reset clears it. Vaillant's own instruction on its fault-code page is “This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.” This is not a code that clears itself, and a boiler that restarts after a reset has not been fixed.

It helps to know what the visit involves, because F25 is a diagnosis job before it is a parts job. An engineer will typically test the flue gas safety temperature limiter and its plug connection, check the cable harness and circuit board connections, test or free the pump, check the diverter valve and any stuck or closed internal valves, bleed air from the internal air vent, check the flue for blockage, excessive length or flue gas recirculation, and run Vaillant's P.0 check programme to read live values. If the system is sludged, a chemical clean or power flush may follow.

F25 covers a wide span of underlying faults, from a sensor connection to a flue or circulation problem, so the work behind it varies a lot. Ask the engineer for the cause in writing, not just the repair, and for the cost before work starts, because the same lockout will return if the underlying circulation or flue problem is left alone.

You can book a one-off repair with us without joining a plan, or call 0333 772 6247. One-off repair visits run Monday to Friday, 09:00 to 17:00. A £95 call-out fee applies and is paid in advance, and the engineer diagnoses the fault and tells you what is needed and what it costs before any work goes ahead. One-off repairs are not covered by a workmanship guarantee. 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.

A Smart Plan boiler and central heating module is a different thing, and it is worth being straight about what it does. It covers parts and labour up to £500 per 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. On a job like a new pump, flue work or a power flush, that gap can be real. Cover is also for future breakdowns, not a fault your boiler is already showing: an F25 on the display today counts as a pre-existing fault, so a new plan would not pay for this one. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Outside those hours you are only seen sooner for a genuine emergency breakdown as set out in your terms, so a Friday-evening or weekend fault can wait until Monday.

A few more things to know before you set anything up. 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. Your cover itself applies from the start of your agreement: the first 30 days are a call-out fee window, not a gap in cover. 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. It comes from UK Boiler Company Ltd, trading since 2014.

One safety note. F25 is a flue gas temperature fault, and the boiler has locked itself out to keep things safe, so leaving it locked out is the right thing to do. A working carbon monoxide alarm is worth having in any room with a gas appliance, and if you do not have one, fit one. 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 F25 fault code FAQs

Boiler locked out on F25? We'll get an engineer out to you.

Book a one-off repair and a Gas Safe registered engineer will diagnose the F25 lockout. One-off repair visits run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, and the engineer tells you what is needed and what it costs before any work goes ahead. One-off repairs are not covered by a workmanship guarantee. We aim to get an engineer out to you within two working days. A Smart Plan boiler and central heating module is for next time rather than this one: cover is for future breakdowns, not a fault your boiler is already showing, so an F25 on the display today counts as a pre-existing fault. It covers parts and labour up to £500 per 12-month period for a boiler under 7 years old, or up to £200 if it is older, and you pay anything above that limit yourself. A larger repair such as a new pump, flue work or a power flush can cost more than the limit. 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, no-access visits and early annual-service requests, though your cover itself applies from the start of your agreement. 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. Smart Plan is a service plan, not insurance. Call 0333 772 6247.

Helpful links

• Vaillant F24 fault code
• Vaillant F23 fault code
• Vaillant F27 fault code
• All Vaillant fault codes
• Vaillant boiler help and guides
• Boiler repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register