Vaillant F56 fault code: a carbon monoxide safety lockout

What does the Vaillant F56 fault code mean?

A Vaillant F.56 fault means the boiler's carbon monoxide sensor measured too much carbon monoxide in the combustion, so the boiler shut itself down and locked out. Vaillant's manual points at the gas valve assembly, its plug or its cable harness. You cannot fix this yourself. Leave the boiler off and book a Gas Safe registered engineer.

Vaillant's installation and maintenance instructions for the ecoTEC plus, document 0020244997_07, an installer and servicing manual rather than a homeowner guide, word it as "Safety switch-off: CO limit exceeded" and tell the engineer to "Check: Gas valve assembly, plug, cable harness. If the fault occurs again after being cleared, the gas valve assembly is defective."

F.56 does not appear on Vaillant's public fault-code page at vaillant.co.uk/service/boiler-fault-codes/, and it is not in the homeowner operating instructions either. It is documented only in the installer and servicing manuals. If you searched Vaillant's own website and found nothing, that is why. Vaillant's homeowner guide covers exactly this case: "If fault codes or limp home mode codes other than those listed are displayed, action by a competent person is required."

This is a deliberate safety shutdown, not a glitch. The boiler is telling you it is no longer burning gas correctly. F56 is the boiler reading its own combustion, not the air in your rooms, so on its own it does not mean carbon monoxide has spread through the house. It is still a reason to leave the boiler off until an engineer has been. If anyone at home feels unwell, if you can smell fumes, or if a carbon monoxide alarm sounds, leave the property and call the National Gas Emergency Service on 0800 111 999.

Key facts

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

• Meaning: the boiler's carbon monoxide sensor, which Vaillant calls the all-gas sensor, measured CO above the safe limit and the boiler locked out. Vaillant's manual wording is "Safety switch-off: CO limit exceeded".
• DIY-fixable? No. The safest thing you can do is leave the boiler off. If you do reset, reset only once, and if F56 returns, stop.
• Engineer job from the start: every check on the list is engineer-only work, including anything that needs the boiler casing removed, plus flue gas analysis and combustion (CO/CO2) checks.
• Models affected: only Vaillant boilers fitted with a CO sensor. F.56 is in the manual for the ecoTEC plus VU and VUW ..6/5-5 range (612, 615, 618, 624, 630, 637, 825, 832, 835, 838) and the ecoTEC exclusive 835 and 843. It is absent from the current ecoTEC plus manual.
• Not on Vaillant's own consumer fault-code page, and not in the homeowner operating instructions. It appears only in the installer and servicing manuals.
• Ignore the sites that call F56 a mass flow sensor fault. That wording belongs to F53. None of the five official Vaillant UK manuals we checked says it about F56.
• If you rent your home, tell your landlord or letting agent now. Gas appliances in a rented home are their legal responsibility under the Gas Safety (Installation and Use) Regulations 1998, so you should not be paying for this repair yourself.
• Cover is for future breakdowns, not a fault your boiler is already showing. An F56 on the display today counts as a pre-existing fault and would not be covered by a new plan, so this one is a one-off repair.
• The cover terms, including the £95 call-out fee, the call-out hours and the 12-month agreement period, are set out further down this page.

Why has my boiler locked out with F56?

Because the sensor that watches the combustion saw more carbon monoxide than the boiler is allowed to produce, and the safety system pulled the plug. The question an engineer then answers is why the burner stopped burning cleanly.

Vaillant's two manuals name the same short list of suspects: the gas valve assembly itself, a contact fault where that valve plugs in, the cable harness running to it, or another component in the combustion regulation system. All four sit behind the casing.

The table below runs through what Vaillant's manuals list, what you might notice from outside the boiler, and who can safely deal with each one.

F56 causes and who fixes them

Can I fix a Vaillant F56 myself?

No, and we would rather say that plainly than pad this section out. F56 is a carbon monoxide safety shutdown, so the useful things you can do are keep everyone safe and gather a few facts for the engineer. Anything that would actually fix it is behind the casing.

Work through these in order.

1. Treat this as a carbon monoxide safety shutdown. The safest thing you can do is leave the boiler off and book a Gas Safe registered engineer. If anyone in the home feels unwell, or you smell fumes, leave the property and call the National Gas Emergency Service on 0800 111 999.

2. Check you have a working audible carbon monoxide alarm near the boiler, and test it. If you do not have one, fit one before the boiler is used again.

3. If you rent your home, tell your landlord or letting agent now. Gas appliances in a rented home are their legal responsibility under the Gas Safety (Installation and Use) Regulations 1998, and you should not be paying for this repair yourself.

4. Check your room thermostat is turned up and calling for heat, so you can tell the engineer whether the boiler was actually being asked to fire when it locked out.

5. Read the pressure gauge on the front of the boiler and write the number down for the engineer. Do not open the casing to do this.

6. Look at the outside of the boiler, and at the flue terminal outside the house, for obvious damage or blockage. Look only. Do not open the boiler casing and do not touch the flue.

7. Check whether your gas hob or other gas appliances still work, so you know whether the gas supply to the whole house is off rather than just the boiler.

8. If you choose to reset, press the reset button once, holding it for longer than three seconds as the Vaillant operating instructions describe. If F56 comes back, stop. Do not reset it again, and call a Gas Safe registered engineer.

Stop at that point, and here is the reason. Repeated resetting on F56 asks a boiler that has already been caught producing too much carbon monoxide to fire again. Vaillant only documents a multi-reset procedure for its ignition fault, not for this one.

The rest is engineer work: testing, adjusting or replacing the gas valve assembly, checking or reseating its plug and socket, inspecting or replacing the cable harness, checking or replacing the CO sensor, flue gas analysis and combustion checks, calibrating the heat generator, and checking the gas flow pressure. Every one of those is engineer-only, and the ones inside the boiler need the casing removed.

Is my code F56, F55 or F53?

They are close together on the display and easy to mix up, so check the number carefully. F55 is "Fault: CO sensor", which means the sensor itself is defective or short-circuited. F56 is "Safety switch-off: CO limit exceeded", which means the sensor is working perfectly and is reporting too much carbon monoxide. F55 is the sensor failing; F56 is the sensor doing its job.

F53 is the one that causes the most confusion online. Vaillant's public page calls F53 a combustion regulation fault, and the ecoTEC plus manual words it "Gas valve assembly mass flow unexpected". Trade sites, WarmZilla among them, have taken that mass flow wording and attached it to F56. None of the five official Vaillant UK manuals we checked does. If you have read that F56 is a mass flow sensor fault, that description belongs to F53.

The other neighbours are different faults again. F54 is "Fault: Boiler start", which Vaillant's public page describes as a fault in the gas supply, seen alongside F28 or F29, meaning no or insufficient gas inlet pressure. F57 is "Fault: Measuring program", a regulation fault caused by a corroded ignition electrode. F61 is "Fault: Fuel valve actuation", where the gas valve assembly cannot be actuated. F62 is "Fault: Fuel valve switch-off delay", a delayed switch-off of the gas valve after the flame goes out.

You may also find a fault-code site describing F56 as a heating circuit or pump fault. No Vaillant manual we checked says that, and the two that list F56 both word it "Safety switch-off: CO limit exceeded".

When should I call a Gas Safe registered engineer?

Now, rather than after a few more resets. With F56, waiting and retrying is the wrong instinct. Vaillant's own homeowner guide says that a code which is not in the consumer table needs action by a competent person, and F56 is not in that table.

Call the National Gas Emergency Service on 0800 111 999 straight away, and leave the property, if anyone at home feels unwell, if you smell gas or fumes, or if a carbon monoxide alarm sounds. A suspected gas or carbon monoxide emergency is not a repair booking and it is not a cover-plan call-out. That call comes first, and the repair is booked afterwards.

For the repair itself, the engineer will run a flue gas analysis and combustion checks, look at the gas valve assembly, its plug and socket and the cable harness, and check the CO sensor. If the fault returns after being cleared, Vaillant's manual says the gas valve assembly is defective.

You can book a one-off repair with us and we aim to get a Gas Safe registered engineer out to you, or call us on 0333 772 6247. Repairs are carried out by a national network of thousands of Gas Safe registered engineers. One-off repair appointments run Monday to Friday, 09:00 to 17:00. A one-off repair is a single paid job rather than cover, it is paid for separately, and it does not carry a workmanship guarantee.

Cover is a different thing, and it is worth being straight about what it does for you today. Cover is for future breakdowns, not a fault your boiler is already showing, so an F56 on the display now counts as a pre-existing fault and would not be covered by a new plan. This repair is a one-off repair. A plan protects you against the next fault.

If you do want that protection, 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, and you pay anything above your cover limit yourself. Cover is modular, so you pick only what you want. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, unless it is a genuine emergency breakdown. We aim to get an engineer out to you within two working days. That is a target for getting someone to your door, not a promise of a finished repair, because parts and second visits are not in anyone's gift.

A few practical things before you set anything up. Outside the call-out hours a booking is made on the next working day, so a Friday evening fault is normally attended the following week, and you should plan to be without heating and hot water in the meantime. If you think yours counts as a genuine emergency breakdown, call 0333 772 6247 and we will assess it. 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 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. It arranges and funds covered repairs up to the limits on your plan rather than paying out a cash sum, and it is provided by UK Boiler Company Ltd, trading since 2014.

Vaillant F56 fault code FAQs

Seeing F56? Leave the boiler off and let us get an engineer to you.

If you smell gas or fumes, if anyone feels unwell, or if a carbon monoxide alarm sounds, call the National Gas Emergency Service on 0800 111 999 before anything else. For the repair, book a one-off repair with us or call 0333 772 6247, and a Gas Safe registered engineer can run the combustion checks and look at the gas valve assembly. Repairs are carried out by a national network of thousands of Gas Safe registered engineers. One-off repair appointments run Monday to Friday, 09:00 to 17:00, and a one-off repair is a single paid job rather than cover, paid for separately and without a workmanship guarantee. Cover is a separate thing, and it is for future breakdowns, so an F56 showing today counts as a pre-existing fault and would not be covered by a new plan. If you want protection against the next fault, a Smart Plan boiler and central heating module funds covered repairs up to £500 a year for a boiler under 7 years old, or up to £200 a year if it is older, and you pay anything above that. It is a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. 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 run Monday to Friday, 08:00 to 18:00 unless it is a genuine emergency breakdown, 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. 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

• Vaillant boiler help and guides
• All Vaillant fault codes
• Vaillant F54 fault code (gas supply fault)
• Vaillant F61 fault code (gas valve actuation)
• Vaillant F62 fault code (gas valve switch-off delay)
• Vaillant fault-code help hub
• Boiler repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register
• HSE: landlords' gas safety duties