Glow-worm F68 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F68 fault code mean?
The F68 code appears when your Glow Worm boiler's control board detects an inconsistent or fluctuating signal from the ionisation probe — the component that confirms a stable flame is present after ignition. Rather than risk unburned gas building up in the combustion chamber, the boiler locks itself out as a safety measure. In practice, you may notice the boiler fires up, burns for a few seconds, then cuts out, often repeating this cycle before eventually displaying F68. This code sits alongside F27 and F87 in Glow Worm's group of flame-sensing faults, all of which point to problems with how the boiler detects and confirms its burner flame.
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 or degraded ionisation probe Common
Over time, carbon deposits and general combustion residue build up on the probe's metal tip, reducing its ability to pass an accurate electrical signal back to the PCB. The boiler may ignite momentarily — the initial heat generates a weak signal — but the control board quickly loses confidence and shuts the gas off. This is the most frequent root cause of F68 and is usually identified and resolved during a service visit.
- Physically damaged ionisation probe Common
The probe is a fragile component: a thin metal rod housed within a ceramic insulator. It can be cracked, bent, or knocked out of position during other repair work or servicing. Even a hairline crack in the ceramic allows the electrical signal to dissipate before reaching the PCB, producing exactly the kind of fluctuating reading that triggers F68.
- Corroded probe contacts or wiring Sometimes
In older boilers or those installed in damp environments, the probe's metal contacts and connecting wires can corrode or develop loose connections. Increased electrical resistance weakens the flame signal, making it appear unstable to the control board even when the burner flame itself is perfectly normal.
- Faulty or sticking gas valve Sometimes
If the gas valve is not opening fully or consistently, the burner receives an irregular gas supply, causing the flame to flicker and fluctuate in intensity. The ionisation probe picks up this physical instability in the flame and reports it as a fluctuating signal. Gas valves that begin misbehaving in this way tend to deteriorate further, so replacement is usually recommended over repair.
- PCB fault Rare
If the probe, its wiring, and the gas valve all check out correctly, attention turns to the PCB — the boiler's main control board. A failing PCB may misinterpret a perfectly good flame signal, or its flame-monitoring circuitry may develop a fault that mimics a probe problem. This is the least common cause but should be considered once other components have been ruled out.
How to fix it
- Reset the boiler DIY safe
On most Glow Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold it there for around 30 seconds, then turn it back to your desired temperature setting. Allow the boiler to complete a full ignition cycle before deciding whether the fault has cleared. If F68 returns within a short time, do not keep resetting — repeated resets without addressing the underlying cause will not fix the problem and can mask useful diagnostic information for an engineer.
- Check that your gas supply is live DIY safe
Try lighting a gas hob or another gas appliance in your home. If those also fail to work, the issue may be with your gas supply rather than the boiler itself — contact your gas supplier. Also check that the isolation valve on the gas supply pipe to the boiler is fully open (the handle should be in line with the pipe). If the gas supply seems fine everywhere else, the problem is most likely within the boiler.
- Do not attempt to access or clean the ionisation probe yourself Gas Safe engineer
Although cleaning the probe sounds straightforward, doing so requires opening the combustion chamber — a sealed area containing gas components. This work must only be carried out by a Gas Safe registered engineer. Incorrect reassembly can result in gas leaks or incomplete combustion, both of which are serious safety hazards.
- Have a Gas Safe engineer inspect and clean or replace the ionisation probe Gas Safe engineer
An engineer will access the combustion chamber, carefully remove the probe, and assess its condition. If carbon fouling is the issue, a careful clean may restore normal operation. If the ceramic insulator is cracked, the rod is bent, or the probe simply fails to produce a reliable signal after cleaning, it will be replaced. This is the most common repair for F68 and is usually straightforward for an experienced engineer.
- Have the engineer check probe wiring and connections Gas Safe engineer
While the combustion chamber is open, the engineer should inspect the wiring harness running from the probe back to the PCB, looking for loose terminals, heat-damaged insulation, or corrosion. A poor connection here can produce exactly the same fluctuating signal as a faulty probe, so this check is essential before condemning the probe itself.
- Have the gas valve assessed if the probe is not the cause Gas Safe engineer
If the probe and its wiring are in good condition, the engineer will test the gas valve's operation — checking that it opens to the correct degree and maintains a consistent gas flow to the burner. A valve that is sticking or not modulating correctly will typically be replaced rather than repaired, as partial fixes rarely provide a lasting resolution.
- Have the PCB tested as a last resort Gas Safe engineer
Should the probe, wiring, and gas valve all be confirmed as functioning correctly, the engineer may use diagnostic equipment to assess whether the PCB is correctly processing the flame signal. PCB replacement is an involved and relatively expensive repair, so a thorough engineer will exhaust other possibilities first. Contact a Gas Safe registered engineer if you have not already done so.
Parts you may need
- Ionisation probe / flame sensor · from £25
- Ignition electrode · from £30
- Probe wiring harness · from £20
- Gas valve · from £120
- PCB (main control board) · from £200
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow Worm F68 fault myself?
The initial steps — resetting the boiler and confirming your gas supply is on — are safe for any homeowner to try. Beyond that, F68 requires work inside the combustion chamber or on gas components, which must legally be carried out by a Gas Safe registered engineer. Attempting to clean or replace the ionisation probe yourself without the correct registration is both dangerous and illegal in the UK.
How much does it typically cost to fix an F68 fault in the UK?
For the most common repair — cleaning or replacing the ionisation probe, including labour — expect to pay roughly £120–£200. If the gas valve also needs replacing, the total is more likely to fall in the £250–£350 range. A PCB replacement is less common but can push costs to £400–£550 in total; if an engineer suspects this from the outset, it is worth getting a second opinion given the expense.
Why does my Glow Worm boiler keep firing and then cutting out before showing F68?
This cycle — ignite, run briefly, shut down — is the classic symptom of a dirty or failing ionisation probe. The probe generates just enough signal when it first heats up to satisfy the control board, but as carbon deposits interfere with the current flow, the signal drops below the threshold the PCB considers reliable. The board cuts the gas as a safety measure, then retries. After several failed attempts, it locks out and displays F68. Cleaning or replacing the probe almost always resolves this pattern.
Will an annual boiler service prevent the F68 fault from recurring?
Regularly — yes. During an annual service, an engineer will inspect and clean the ionisation probe, check electrode condition, and test the gas valve operation. Catching a building carbon deposit on the probe before it causes a full lockout is exactly the kind of preventive work a service is designed to do. An annual service typically costs £80–£120 in the UK, which is often less than a single call-out fee for a breakdown repair.