Vaillant F.87 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.87 fault code mean?
The F.87 code on a Vaillant boiler indicates a safety lockout triggered by incorrect flame detection — specifically, the boiler's control board is sensing the presence of a flame at a point when no flame should exist (i.e. when the gas valve is closed). Because this suggests a potentially dangerous situation, the boiler shuts itself down as a precaution. The fault broadly falls within the F.87–F.89 range of electrode and gas valve component faults. In practice, the root cause is almost always a worn or corroded ignition electrode, moisture-affected electronics, or in more serious cases a faulty PCB or gas valve regulation problem. The boiler will not restart on its own — a manual reset is required, and a Gas Safe engineer will typically be needed to diagnose and resolve the underlying issue.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Corroded or worn ignition electrode Common
Over time, the ignition electrode degrades — particularly in older boilers (those manufactured before around 2011). Corrosion can interfere with the electrode's ability to accurately signal flame presence or absence to the control board, causing the boiler to incorrectly detect a flame when the gas valve is off. This is the most frequently seen root cause of F.87.
- Faulty ignition lead or spark generator Common
The lead connecting the electrode to the spark generator can crack, fray, or develop high resistance over time. A damaged ignition lead or failing spark generator can send erroneous signals to the PCB, mimicking a flame detection fault even when the electrode itself is in reasonable condition.
- Moisture on the PCB or electronics Sometimes
Condensation or water ingress onto the control board or surrounding electronics can cause spurious voltage readings that the boiler interprets as a flame signal. This is more common in boilers located in unheated spaces or where a condensate pipe has previously leaked or blocked. The spark generator circuit is particularly susceptible.
- Faulty PCB (printed circuit board) Sometimes
If the control board itself has developed a fault — whether due to age, power surge, or moisture damage — it may misread signals from the electrode circuit entirely. A faulty PCB will often trigger F.87 after other components have been checked and found to be in good working order.
- Gas valve regulation fault Rare
An issue with how the gas valve opens and closes — for example, if it fails to close fully — could contribute to the boiler detecting residual combustion when it should not. This is a less common cause but should be assessed if electrode and PCB checks come back clear.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (usually marked with a flame symbol or a rotating arrow) for around three seconds. The boiler may attempt a restart. If F.87 returns after one or two resets, do not continue resetting — repeated failed lockouts indicate a component fault that needs professional attention. Resetting more than two or three times without success will not fix the underlying problem and could mask important diagnostic information.
- Check your gas supply is active DIY safe
Confirm that other gas appliances in the property (such as a gas hob or fire) are working normally. If no gas appliances are functioning, contact your gas supplier, as this would indicate a supply issue rather than a boiler fault. If other appliances work fine, the gas supply to the boiler is likely not the issue.
- Have a Gas Safe engineer inspect the ignition electrode Gas Safe engineer
An engineer will remove the electrode assembly and check for visible corrosion, cracking, or carbon deposits on the tip. A heavily corroded or damaged electrode must be replaced with a manufacturer-approved part. This is the most common fix for F.87 and is relatively straightforward for a qualified engineer.
- Test the ignition lead and spark generator Gas Safe engineer
The engineer will check the ignition lead for continuity and inspect it for cracks or damage to the insulation. The spark generator will also be tested for correct output. Either component can be replaced independently if found to be faulty, often at lower cost than a PCB replacement.
- Inspect the PCB and electronics for moisture damage Gas Safe engineer
The engineer will look for signs of water ingress, corrosion, or burn marks on the control board and surrounding wiring. If moisture is found, the affected components may need to dry out and be cleaned, or replaced entirely. A faulty PCB will need to be swapped for a new unit — this is one of the more costly outcomes of an F.87 fault.
- Assess gas valve operation Gas Safe engineer
If all ignition and PCB components check out, the engineer will assess whether the gas valve is closing fully and operating within specification. A valve that fails to close correctly can cause residual flame signals. Gas valve work must only ever be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If you have reset the boiler and confirmed the gas supply without success, book a Gas Safe registered engineer to carry out a full diagnostic. F.87 is not safely resolvable without opening the boiler casing and testing internal components. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Ignition electrode · from £35
- Ignition lead · from £25
- Spark generator · from £45
- Vaillant PCB (control board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £90–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.87 myself?
Not safely, no. The only homeowner actions worth trying are a boiler reset (once or twice) and confirming your gas supply is live. Beyond that, diagnosing and replacing components like the ignition electrode, spark generator, or PCB requires the boiler casing to be opened — and any work on internal gas boiler components must be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998.
How much does it cost to fix a Vaillant F.87 fault?
For the most common repairs — replacing the ignition electrode, ignition lead, or spark generator — you'd typically expect to pay somewhere between £90 and £320 including labour. If the PCB turns out to be the cause, parts alone can cost £250–£400 or more, pushing the total bill higher. If your boiler is more than 10–12 years old and the PCB has failed, it's worth getting a new boiler quote alongside the repair quote, as the economics may favour replacement.
Why does F.87 tend to affect older Vaillant boilers more?
Vaillant boilers manufactured before around 2011 are more susceptible to F.87 because the ignition electrode and surrounding components naturally degrade with age. Corrosion on the electrode tip becomes more pronounced over time, and older circuit boards are more vulnerable to moisture damage. Regular annual servicing can catch electrode wear before it leads to a lockout, which is why F.87 is more often seen on boilers that have missed services.
My Vaillant boiler keeps showing F.87 even after the engineer replaced the electrode — what else could it be?
If the electrode has been replaced and F.87 returns, the next most likely culprits are the ignition lead, the spark generator, or moisture on the PCB. In some cases, the PCB itself has developed a fault and is misreading the electrode signal even with a new electrode in place. A thorough engineer will test each component in sequence rather than replacing parts speculatively — if you're being quoted for a PCB straight away without those checks, it's reasonable to ask whether the lead and spark generator have been ruled out first.