Glow-worm F87 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F87 fault code mean?
The F87 code appears when your Glow-Worm boiler is unable to confirm that a flame is actually burning inside the combustion chamber. The boiler relies on an ionisation (flame-sensing) electrode to detect the presence of a flame by measuring a tiny electrical current through it. When this signal is absent or too weak — whether because the electrode itself is faulty, its wiring is compromised, or the gas supply to the burner is unstable — the boiler locks out as a safety precaution to prevent unburned gas from accumulating. Glow-Worm shares its engineering platform with Vaillant, and F87 sits within a cluster of flame-detection faults (alongside F27 and F68) that all point to the boiler losing confidence in the burner flame. A single reset attempt is reasonable, but if the lockout returns immediately, a Gas Safe registered engineer is required.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Dirty, corroded, or cracked ionisation electrode Common
Over time, the flame-sensing electrode accumulates carbon deposits or corrodes, which stops it from passing the small electrical current needed to confirm a flame is present. A cracked ceramic electrode body can also allow the signal to earth out incorrectly. This is the most common root cause of F87 and is often picked up during a long-overdue annual service.
- Faulty ignition lead or poor electrode connection Common
The lead connecting the electrode to the PCB can deteriorate, chafe against the casing, or simply work loose at either end. A broken or poorly seated lead produces the same symptom as a failed electrode — the PCB receives no flame confirmation signal and locks the boiler out.
- Gas valve regulation problem Sometimes
If gas is reaching the burner at the wrong pressure or flow rate, the flame may be too weak or erratic for the electrode to detect reliably. The gas valve could be sticking, worn internally, or have a faulty solenoid, producing a marginal or unstable flame that causes intermittent F87 lockouts.
- PCB (printed circuit board) fault Rare
The PCB both sends the high-voltage spark for ignition and reads the ionisation signal back from the electrode. If the PCB's flame-detection circuitry is damaged — perhaps due to a power surge or moisture ingress — it may misread or fail to read a perfectly healthy electrode. This is typically only confirmed after all other components have been ruled out.
- Blocked or partially obstructed condensate pipe Rare
Although not a direct cause of F87, a frozen or blocked condensate pipe can interfere with normal boiler operation and may contribute to combustion issues. An engineer will check this as part of a thorough diagnostic.
How to fix it
- Check your gas supply is working DIY safe
Before doing anything else, confirm gas is reaching your home by testing another gas appliance — a hob or gas fire, for example. If those aren't working either, the issue is with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 if you suspect a supply problem or smell gas.
- Attempt a single boiler reset DIY safe
On most Glow-Worm models, rotate the CH control knob to the reset position (marked with a power/restart symbol), hold it there for a few seconds, then turn it back to your desired temperature. Wait up to 60 seconds for the boiler to attempt ignition. If it fires up and runs normally, monitor it over the next few hours. If F87 returns straight away, do not keep resetting — repeated forced ignition attempts can stress the PCB and burner components. One reset is fine; a second failed attempt means it is time to call an engineer.
- Check the system pressure (if the boiler also shows low pressure) DIY safe
If your pressure gauge reads below 1 bar, low pressure may be compounding the fault. Top up via the filling loop until the gauge reads between 1 and 1.5 bar, then try the reset again. However, if the pressure is normal and F87 persists, this step is not relevant to the fault.
- Have a Gas Safe engineer inspect and test the ionisation electrode Gas Safe engineer
The engineer will remove the electrode, check for carbon fouling, cracks, or corrosion, and test its electrical continuity. In many cases a thorough clean resolves the issue on an otherwise healthy electrode. If the electrode is cracked or worn beyond cleaning, it will be replaced — a straightforward repair that usually resolves F87 in one visit.
- Have the engineer check the ignition lead and all associated wiring Gas Safe engineer
The engineer will inspect the lead from the electrode to the PCB for signs of damage, chafing, or a loose connector. They will also check the electrode's earth path and ensure the probe is correctly positioned relative to the burner. A damaged lead is inexpensive to replace and often overlooked.
- Have the engineer test the gas valve Gas Safe engineer
The engineer will check gas pressure at the valve and measure its solenoid resistance (typically 50–54 ohms ±10% on Vaillant-platform boilers). A valve that fails this test, or that is not delivering the correct gas rate to the burner, will need replacing. If the valve passes, the diagnostic focus shifts to the PCB.
- Have the engineer assess the PCB if all other components are healthy Gas Safe engineer
If the electrode, leads, and gas valve all check out, the PCB's flame-detection circuitry is the remaining suspect. The engineer will confirm this with specialist diagnostic equipment before recommending replacement, as a new PCB is the most expensive repair associated with F87.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All work on combustion components, electrodes, gas valves, and PCBs must be carried out by a Gas Safe registered engineer. You can find one at the Gas Safe Register website (gassaferegister.co.uk). Always ask for their Gas Safe ID card before any work begins.
Parts you may need
- Ionisation / flame-sensing electrode · from £25
- Ignition electrode (combined spark and sense, where applicable) · from £35
- Ignition lead / HT lead · from £20
- Gas valve · from £130
- PCB (printed circuit board) · from £220
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 £130–£380, depending on the underlying cause.
Frequently asked questions
Is F87 dangerous — should I leave the house?
F87 is a safety lockout, which means the boiler has already shut itself down to prevent a hazard. You do not need to evacuate unless you smell gas. If you do detect a gas smell, leave the property immediately, avoid operating any switches or flames, and call the National Gas Emergency Service on 0800 111 999. If there is no gas smell, the boiler is simply locked out and safe to leave in that state while you arrange an engineer.
Can I keep resetting the boiler to get hot water while I wait for an engineer?
One reset attempt is fine; two at most if the first appeared to work briefly. Repeatedly forcing the boiler to attempt ignition when it has a genuine flame-sensing fault puts extra strain on the PCB and ignition components and could turn a relatively cheap electrode replacement into a more expensive PCB repair. It is better to leave the boiler locked out and use an electric shower or immersion heater in the interim.
Why has my Glow-Worm only just developed F87 — it was serviced last year?
Electrodes degrade gradually and can fail between services, particularly on boilers that run heavily through winter. Carbon deposits build up faster in boilers with combustion that is slightly off-tune, and corrosion can accelerate if there is any moisture ingress around the heat exchanger. If F87 has appeared less than 12 months after a service, it is worth asking your engineer whether the electrode was cleaned or checked at that visit.
How much does it typically cost to fix a Glow-Worm F87 fault?
For the most common repair — cleaning or replacing the ionisation electrode and lead — expect to pay roughly £130–£200 including parts, labour, and the engineer's call-out. If the gas valve is at fault, the total usually rises to £280–£380. A PCB replacement is significantly more expensive (often £450–£700 all-in) but is relatively rare for this fault code; if your engineer diagnoses a PCB fault on a boiler over 10 years old, it is worth getting a quote for a replacement boiler as a comparison.