Glow-worm F86 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F86 fault code mean?
The F86 code appears when your Glow-Worm boiler detects an open or broken circuit at the underfloor heating safety contact connection. On the boiler's PCB there is a dedicated terminal (labelled 'B.off' or X41) that expects either a connected underfloor heating overheat thermostat or, where no underfloor heating is present, a short link wire to complete the circuit. If that circuit is open — because the link wire is missing, a thermostat has tripped or failed, or wiring has come loose — the boiler locks out and shows F86. This fault applies across all Glow-Worm boiler models.
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
- Missing or disconnected link wire at terminal X41 (B.off) Common
On boilers that are not connected to an underfloor heating system, a short link wire must bridge the B.off terminal (X41) to complete the circuit. If this wire was never fitted during installation, has worked loose over time, or has been inadvertently removed during other work, the boiler will immediately lock out with F86. This is by far the most frequent cause on systems without underfloor heating.
- Tripped or failed underfloor heating overheat thermostat Common
Where an underfloor heating system is connected to the boiler, the safety thermostat wired into the B.off terminal may have tripped due to an overheating event in the underfloor circuit, or the thermostat itself may have developed an internal fault and is no longer completing the circuit. The thermostat may need resetting or replacing.
- Loose or damaged wiring between the thermostat and PCB Sometimes
Corroded terminals, chafed cable insulation, or a connection that has vibrated loose along the wiring loom can interrupt the circuit between the B.off terminal and the underfloor thermostat (or link wire). The fault can appear intermittently before becoming permanent.
- PCB fault Rare
In a small number of cases the PCB itself develops a fault that causes it to misread or fail to recognise a correctly wired B.off circuit. PCB failure is relatively rare and is usually only diagnosed once all wiring and thermostat checks have been completed.
How to fix it
- Attempt a boiler reset DIY safe
On most Glow-Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold for a couple of seconds, then return it to your desired temperature setting. Wait two to three minutes to see whether the boiler fires successfully. You can attempt this up to two or three times. If F86 returns each time, there is an underlying fault that a reset alone will not cure — move on to calling an engineer.
- Check whether your boiler is connected to an underfloor heating system DIY safe
Consider whether your home has underfloor heating that is plumbed into the boiler. If it does not, the likely culprit is simply a missing link wire — a straightforward fix for an engineer. Pass this information on when you call, as it helps the engineer arrive with the right materials and speeds up diagnosis.
- Engineer to inspect terminal X41 (B.off) and check or fit the link wire Gas Safe engineer
A Gas Safe registered engineer will open the boiler casing and inspect the B.off terminal on the PCB. If no underfloor heating is installed, they will verify that a suitable link wire is properly seated and make or replace the connection as needed. This is often the quickest and least expensive resolution for F86.
- Engineer to test the underfloor heating overheat thermostat and its wiring Gas Safe engineer
Where an underfloor heating system is present, the engineer will check whether the safety thermostat has tripped and whether it can be reset, or whether it has failed and needs replacing. They will also inspect the wiring loom from the thermostat back to terminal X41, looking for loose terminals, corrosion, or damaged insulation.
- Engineer to test the PCB if wiring checks are inconclusive Gas Safe engineer
If the link wire, thermostat, and associated wiring all check out correctly but F86 persists, the engineer will carry out PCB diagnostics. A faulty PCB will need to be replaced; this typically takes one to two hours once the correct replacement board is available.
- Call a Gas Safe registered engineer Gas Safe engineer
If resetting the boiler has not resolved F86, contact a Gas Safe registered engineer. Only a registered engineer is legally permitted to work on boiler components, wiring, and the PCB. Have your boiler model, installation date, and any history of previous faults ready to share — it will help them diagnose efficiently.
Parts you may need
- PCB (main control board) · from £280
- Underfloor heating overheat thermostat · from £55
- Wiring loom / harness · from £45
- Link wire (B.off terminal) · from £5
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 the Glow-Worm F86 fault myself?
The only action a homeowner should take is attempting a boiler reset — no more than two or three times. The underlying cause of F86 involves internal wiring, PCB terminals, or a connected thermostat, all of which must be inspected and repaired by a Gas Safe registered engineer. Attempting to open the boiler casing or interfere with the wiring is unsafe and may invalidate any remaining warranty.
My boiler has no underfloor heating — why am I getting F86?
This is actually the most common scenario for F86. When no underfloor heating system is connected, the B.off terminal on the PCB must be bridged with a link wire to tell the boiler the safety circuit is complete. If this wire was never fitted at installation, or has worked loose over the years, the boiler will lock out with F86. An engineer can fit or re-seat the link wire in a short visit, making this one of the quicker and cheaper fixes for this fault.
How much does it cost to fix Glow-Worm F86?
For most people, the repair falls between £90 and £320. If the cause is simply a missing or loose link wire, you're likely looking at a standard call-out and a short labour charge. A failed underfloor thermostat adds parts cost but still sits within that range. PCB replacement is much less common and significantly more expensive — typically £400 to £650 including parts and labour — so if your engineer suspects the PCB, it is worth comparing that cost against a new boiler installation, especially if the boiler is more than ten years old.
Will resetting the boiler clear F86 permanently?
A reset may temporarily clear the fault if it was triggered by a one-off thermostat trip, but if the root cause — a missing link wire, a failed thermostat, or a wiring fault — is still present, F86 will return. Repeated resets without addressing the underlying issue can mask the fault and potentially cause further wear, so it's best to get the boiler inspected after the fault reappears.