Glow-worm F.68 fault codewhat it means and how to fix it
The flame is burning but its signal is unstable. Glow-worm list air in the gas, low gas pressure, a wrong combustion setting or injector, a poor electrode connection or a blocked condensate pipe.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.68
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.68 means
Glow-worm call F.68 "Fault: Unstable flame (analogue input)" and, on the two-digit boilers, "fluctuation of flame signal". The burner lit and is being held, but the ionisation current the electrode reports is wandering rather than steady, and after enough of that the board gives up.
The published causes are "air in gas, gas flow pressure too low, incorrect air ratio, incorrect gas injector, ionisation flow interruption (cable, electrode)", and the Compact manual adds flue gas recirculation and a blocked condensate discharge pipe, and against the air-in-gas cause it tells the engineer to "reset the unit once" — the one place Glow-worm sanction a reset for this code, and only in the installer's document. That is what F.68 often is after a gas supply has been off: air in the pipe gives a ragged flame for the first burn and a single reset clears it. The other causes are not like that. A combustion setting that has drifted, an injector left from a different gas type, or an electrode with a poor connection produce a flame that is permanently marginal, and the boiler will keep finding it.
The Betacom2 manual gives the engineer the same checks it gives for F28 — gas circuit, flame picture and CO2, igniter connections, electrode condition — which tells you how Glow-worm think of F.68: the ignition fault that nearly worked. It is a combustion code, and combustion adjustment on a Glow-worm is an analyser job for a Gas Safe registered engineer.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Air in the gas supply | After a meter change, a supply interruption or work on the pipework. The Compact manual's remedy is a single reset, and if the flame steadies the fault was never in the boiler. | Common |
| Gas pressure low at the boiler | Glow-worm's second cause. A supply that sags at peak demand, a long undersized run or a partly closed valve leaves the burner hunting, and the signal fluctuates with it. | Common |
| Combustion ratio or injector wrong | The CO2 setting has drifted or the wrong injector is fitted. The Betacom2 manual tells the engineer to observe the flame picture and check the CO2; that needs an analyser. | Occasional |
| Electrode or lead connection poor | Glow-worm's "ionisation flow interruption (cable, electrode)". A corroded terminal or a cracked ceramic gives a signal that comes and goes with vibration and heat. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£400
Time on site
1 hour
Combustion check and adjustment £80–£150. Electrode £90–£180. Gas valve £250–£400 if the pressure regulation has failed.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Glow-worm ignition and flame-sensing electrode
£18–£45Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm gas valve assembly
£90–£200Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: ignition and gas supply
- F1Get it checkedOn Ultracom, Flexicom and Ultrapower boilers F1 is an ignition lockout after five failed attempts, and Glow-worm's guides let you reset it. On Energy and Ultimate boilers F.01 is the return sensor instead.Related: ignition and gas supply
- F.29Get it checkedThe burner lit and then the flame went out while running. Glow-worm blame the gas supply, flue gas recirculating, earthing, the spark transformer and, on newer manuals, a blocked condensate pipe.Related: ignition and gas supply
- F.61Get it checkedThe board cannot drive the gas valve properly — a short in its wiring, coils shorted to earth, or the electronics. The valve stays shut until an engineer has traced it.Related: ignition and gas supply
- F.27Stop — call an engineerThe boiler is sensing a flame when there should not be one. Glow-worm list moisture on the electronics, a failed flame monitor or a leaking gas valve. Not a reset job.Related: ignition and gas supply
- F.62Stop — call an engineerThe gas valve did not shut off when told to — the flame signal lingered after the close command. Glow-worm list a delayed or leaking valve or the electronics. Switch off; call an engineer.Related: ignition and gas supply
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.68
Can I reset it once?
Glow-worm's Compact installation manual says a single reset is the measure for air in the gas, and the user guides reserve the code for a competent person. One reset after a known supply interruption is reasonable; a second is not.
Is F.68 the same fault as F.28?
Cousins. F.28 is a flame that never established; F.68 is one that did but cannot hold a steady signal. Glow-worm give the engineer almost the same checklist for both on the Betacom2.
Could it be the condensate pipe?
The Compact manual lists a blocked condensate discharge pipe against F.68, F.28 and F.29 alike. In freezing weather the outdoor pipe is worth a look before anyone is called.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.