Viessmann F4 fault code: no flame signal explained

What does the Viessmann F4 fault code mean?

The Viessmann F4 fault code means the boiler got no flame signal at ignition. The burner control unit opened the gas valve for its safety time, watched the ionisation electrode, saw no flame, shut the gas off and locked out. The boiler is not broken, and it is not dangerous. It locked itself out on purpose, which is the safety system working.

F4 is a failure to light at burner start, not a flame that dropped out part way through a run. Viessmann records a flame lost during operation as a separate code, so the two are not interchangeable, whatever some third-party guides suggest.

You will see the code written both ways: your display shows F4, plenty of guides pad it out to F04, and both mean the same lockout. What you will not find is F4 on Viessmann's own consumer fault code checker, which covers a set of other codes but not this one. F4 sits in the installation, service and operating manuals instead, which is why owners search the manufacturer's site and come away with nothing.

Key facts

The quick version, before the detail.

• Meaning: no flame signal. The boiler ran an ignition attempt and saw no flame inside the safety time.
• F4 and F04 are the same code. The display shows F4; the padded form is how people write and search it.
• Viessmann does not publish F4 for consumers: it is missing from the public fault code checker, and sits in the manuals instead.
• Homeowner checks: the gas supply, the condensate pipe, the flue terminal from ground level, and one reset.
• Engineer job if it returns after that reset: electrodes, ignition leads, gas pressure, the gas valve.
• Gas emergency? Call 0800 111 999 only if you can smell gas or your carbon monoxide alarm sounds.
• Topping up the pressure will not clear an F4. The code reports a missing flame signal, not a water pressure problem.
• Cover is for future breakdowns. An F4 showing on your boiler today counts as a pre-existing fault and is not covered, so book a one-off repair for that.
• Smart Plan boiler and central heating cover: 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. Cover is modular, so you pick only what you want, and the limits apply to the modules you have chosen.
• 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. Cover still applies in those 30 days, but a call-out inside them costs the £95 fee in advance plus payment for the works in advance.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, unless it is 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 does the F4 code appear?

F4 lands at one of three points in the ignition sequence: the spark never happens, the gas valve never opens, or the boiler lights but cannot sense the flame it has made. Either no gas arrived, no spark arrived, or the flame-sensing side failed.

The checks that follow from those three points are the ones an engineer works through: the ignition and ionisation electrodes, their cables and adjustment, the flame-sensing current, the gas supply pressure, the gas valve and its control voltage, the ignition module, the condensate drain, and air in the gas line. A boiler left set for the wrong gas type after an LPG conversion, cracked electrode insulation and a failed burner control unit sit on the same list.

Two are worth flagging for a British winter. A condensate pipe that cannot drain freely backs up into the boiler and stops the burner, which is why lockouts like this cluster in a cold snap. And a blocked flue terminal or air intake starves the burner, so leaves, snow and nesting material outside are worth a look before anyone opens the case.

What the boiler does in the seconds before it locks out narrows the cause down further. Repeated clicking with no whoosh of ignition points at the spark or the gas side. A whoosh followed by a lockout a second or two later points at flame sensing, because the boiler lit and then lost sight of its own flame. Silence, with no clicking at all, points at the ignition circuit or the control board. A lockout that only happens on the coldest mornings points at the condensate pipe. Tell whoever attends which of those you heard, because it saves diagnosis time on the day.

F4 causes and what to do

What can I safely check myself?

A few checks are worth making before you book anyone. They are the ones that stay outside the boiler case. Anything that opens the case or touches gas is off the list.

1. Read the code off the display and write it down, along with the model number from the data plate. Whoever attends will ask for both.

2. Confirm gas is reaching the house. See whether another appliance such as the hob will light, check the emergency control valve lever at the meter is in line with the pipe, and check a prepayment meter for credit.

3. In cold weather, look at the external condensate pipe, since a frozen or blocked pipe stops the burner. If it is reachable from the ground, warm water poured slowly along it is a recognised homeowner action. Never use boiling water, and leave a pipe at height alone.

4. Look at the flue terminal outside, from ground level only. Clear loose leaves or snow from around the terminal, never reach into it and never push anything into it. Report anything else you find, such as a nest, packaging or damage, to the engineer instead of touching it. If the terminal is at height, leave it alone. Never use a ladder to reach a flue.

5. Before you reset, stop and check one thing: if you can smell gas or your carbon monoxide alarm is sounding, do not press reset. Leave the boiler alone, open windows, keep away from switches and naked flames, and call the National Gas Emergency Service on 0800 111 999. Otherwise, reset once and only once. Press and hold the reset button for about two seconds, and the normal display returns if the fault has gone. Do not reset a burner fault over and over in quick succession, because a fault left unrectified is not something to keep clearing off the screen. If F4 returns, book an engineer.

One check will not help: the pressure gauge. F4 reports a missing flame signal rather than a water pressure problem, so topping up will not clear it.

When to call a Gas Safe registered engineer

If F4 returns after a single reset, it needs a Gas Safe registered engineer. Everything left on the F4 list sits inside the case or on the gas side: measuring the flame-sensing current, adjusting or replacing the electrodes, checking the ignition lead, testing the gas inlet working pressure, testing the gas valve, testing the ignition module, correcting the gas type after a conversion, and replacing the burner control unit.

Give whoever attends the code and the model number from the data plate.

It helps to know what the visit looks like, so you can tell whether the job is being done properly. The engineer takes the case off, checks the ignition and ionisation electrodes for cracked insulation, soot and the right gap, measures the flame-sensing current with the burner running, and checks the ignition lead and its connections. Next comes the gas inlet working pressure at the boiler, then the gas valve and the ignition module. Only when all of that reads correctly does the burner control unit itself come into the frame. Ask them to tell you which part they have diagnosed, and what it costs, before anything is ordered.

One safety point overrides everything above. If you smell gas, do not reset the boiler. Turn the gas off at the meter if you can reach the control safely, open windows, keep away from switches and naked flames, and call the National Gas Emergency Service on 0800 111 999 straight away. The same goes if you suspect carbon monoxide.

For the F4 in front of you now, book a one-off repair with us, or call 0333 772 6247. One-off repair appointments run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, the engineer confirms the cost before any further work goes ahead, and one-off repairs do not carry a workmanship guarantee.

Cover is a separate route, and it is for future breakdowns. An F4 showing on your boiler today counts as a pre-existing fault and is not covered, so book a one-off repair for that. For next time, a Smart Plan boiler and central heating 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 the limit is yours to pay. Cover is modular, so you pick only what you want, 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 your terms, so a Friday-evening or weekend fault can wait until Monday. 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, which is a target for attendance rather than a promise of a finished repair. In your first 30 days a call-out costs the £95 fee in advance plus payment for the works in advance, and that same £95 fee, paid in advance, also applies 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.

Viessmann F4 fault code FAQs

Seeing F4? We'll get a Gas Safe engineer to you.

For an F4 on the display today, book a one-off repair and a Gas Safe registered engineer will look at the ignition, electrodes and gas side. One-off repair appointments run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, the engineer confirms the cost before any further work goes ahead, and one-off repairs do not carry a workmanship guarantee. A Smart Plan boiler and central heating module is for future breakdowns instead, so it will not cover the F4 you are looking at now: 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 older, with cover applying only to the modules you pick. 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 that ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. It is a service plan, not insurance.

Helpful links

• All Viessmann fault codes
• Boiler fault codes by manufacturer
• No heating? Book a repair
• Boiler repair
• Boiler & central heating cover
• More boiler guides on the blog
• Viessmann's official fault code checker
• Check an engineer on the Gas Safe Register