Vaillant F64 fault code: electronics or sensor fault explained

What does the Vaillant F64 fault code mean?

The Vaillant F64 fault code means the boiler's electronics have detected a fault with a sensor circuit, and it locks out rather than run on a reading it can't trust. On the newer ecoTEC plus 600 series, Vaillant's manual describes F64 as the all-gas sensor being defective or having short-circuited. On other ecoTEC plus models, the same code is described instead as an electronics/NTC fault, caused by a short circuit in the flow or return temperature sensor or a fault in the electronics reading it.

That difference matters. F64 does not have one fixed meaning across every Vaillant boiler, so the wording in your own boiler's manual is the one to trust rather than a single blanket description. F64 also doesn't appear on Vaillant's public fault-code list, which jumps from F.62 straight to F.70, so it's a manual-only code. Check the installation and operating manual for your exact model, or have an engineer confirm it when they visit.

Key facts

The quick version, before the detail.

• Meaning: an electronics or sensor fault, but the exact wording and the sensor named differ by boiler model and generation.
• DIY-fixable? Only lightly. Check your fuse box, your system pressure, then reset once. No sensor, wiring or PCB work.
• Gas Safe job if: the sensor, wiring or the PCB needs testing or replacing, or the code persists after a reset.
• Not on Vaillant's public fault-code list: it jumps from F.62 to F.70. F64 is documented only in model-specific manuals.
• Smart Plan boiler & 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.
• 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.
• 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, so a Friday-evening or weekend fault can wait until Monday.
• 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 once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why does the F64 code appear?

F64 points at a sensor circuit or the electronics reading it, not the gas or water side. The likely causes are a short-circuited or faulty flow or return temperature sensor (NTC), or on ecoTEC plus 600-series boilers the combined all-gas combustion sensor, a fault in the wiring harness or an earthing problem, or a defective main PCB. Age-related wear has also been reported by installers, though that isn't one of Vaillant's own listed causes.

The table below runs through the likely causes, what you'll notice, and who can safely deal with each one.

F64 causes and who fixes them

Can I fix a Vaillant F64 myself?

There are a few safe checks you can make before booking anyone. None of them involve opening the boiler casing or testing the sensor, that's the line you never cross. Work through them in this order.

1. Check your fuse box. If the boiler's circuit has tripped, restoring the power and resetting once may clear the fault.

2. Check your system pressure gauge. If it's low, top it up carefully via the filling loop, following your Vaillant manual.

3. Reset the boiler once, following the steps in your Vaillant manual. Reset only once.

4. Note whether F64 comes straight back or only after a delay, that detail helps whoever diagnoses it.

Stop there. Testing the sensor, opening the casing or replacing the PCB is restricted work. Vaillant's own guidance is that aside from very minor fixes, all faults should be looked at by a Gas Safe registered engineer. If F64 comes back after these checks, book an engineer rather than resetting again.

Why F64 doesn't mean the same thing on every Vaillant

F64's exact wording depends on which Vaillant boiler you have. On the newer ecoTEC plus 600 series, Vaillant's manual names it as the all-gas sensor being defective or short-circuited. On the ecoTEC plus 400 series and the older Turbomax Plus range, the same F64 code instead points to the flow or return NTC sensor, or the electronics reading it. The underlying idea is the same, a sensor circuit or the electronics behind it has failed, but the exact component named is not.

That's also why F64 isn't on Vaillant's own public fault-code page, which jumps from F.62 to F.70. It's a manual-only code, so the surest way to know exactly what your boiler means by F64 is to check the installation and operating manual for your model, available via Vaillant's professional download pages, or have a Gas Safe registered engineer confirm it on their visit.

F64 appearing with F27

F64 has been reported alongside the F27 fault code in at least one documented case, traced back to a leaking capacitor on the main PCB that was only resolved by replacing the whole board. That's a single reported case rather than a rule, so don't assume the two always appear together, but if you're seeing both codes it's worth mentioning to whoever diagnoses it. Our Vaillant F27 guide explains that code in more detail.

When to call a Gas Safe registered engineer

If checking your fuse box and system pressure, then a single reset, don't clear F64, or it keeps coming back, the sensor, wiring or electronics need checking by a Gas Safe registered engineer. Vaillant's own guidance is that aside from very minor fixes, all faults should be looked at by a Gas Safe registered engineer, and sensor or PCB work sits well outside a homeowner check.

A fault that needs a new sensor or a main PCB is a bigger job than a simple reset, so the parts and labour can add up. That's worth weighing against how you'd rather cover it, a one-off repair or an ongoing module.

You can book a one-off repair with us and a Gas Safe registered engineer will come and fix it, or call us on 0333 772 6247. 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's over 7. Cover is modular, so you only pick what you want.

A few things worth knowing before you set up cover. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, if the engineer can't get access, and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00; outside those hours you're only seen sooner for a genuine emergency breakdown as set out in your terms, so a fault on a Friday evening or over the weekend can wait until Monday. And using any module starts a 12-month agreement period from the first time you use a service, there's 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. In plain terms, it's a membership that arranges and funds covered repairs up to the limits on your plan, rather than a regulated financial product, it doesn't pay out a cash sum, and cover is limited to the modules you've chosen and the limits set out in your terms. It's provided by UK Boiler Company Ltd, which has traded since 2014 and looked after over 15,000 customers.

One safety note: if you ever smell gas or suspect carbon monoxide, don't wait for an engineer, call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F64 fault code FAQs

Seeing F64 on your Vaillant? We'll come and fix it for you.

Book a one-off repair and a Gas Safe registered engineer will check the sensor and electronics, or set up a Smart Plan boiler and central heating module so covered repairs are handled up to your cover limit, up to £500 a year for a boiler under 7 years old, or up to £200 a year if it's older. It's 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 doesn't cover, no-access visits and early annual-service requests. Cover-plan call-outs are Monday to Friday, 08:00 to 18:00 unless it's a genuine emergency. Using a module starts a 12-month agreement period once you first use a service; leaving early costs the remaining months or 75% of the outstanding balance.

Helpful links

• All Vaillant fault codes
• Vaillant F27 fault code
• Vaillant fault codes help
• Book a boiler repair
• Boiler & central heating cover
• Check an engineer on the Gas Safe Register