Glow-worm F16 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F16 fault code mean?
The F16 code on a Glow-Worm boiler signals that the boiler's control system has attempted to light the burner but cannot confirm a flame is present. Without that confirmation, the boiler locks out as a safety precaution and will not fire again until the underlying fault has been identified and corrected. In plain terms: the boiler tried to ignite, something went wrong at the flame-sensing stage, and it has shut itself down. On certain Glow-Worm models the same code can also be triggered by a fan or wiring fault that causes overheating, so the display reading F16 does not always mean the flame probe itself is to blame — a proper diagnosis by a Gas Safe engineer is needed.
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
- Dirty or worn ionisation/flame probe Common
The ionisation probe sits in the burner flame and passes a tiny electrical current to confirm ignition. Over time, carbon deposits or oxidation build up on the probe tip, reducing its ability to sense the flame reliably. This is the single most common trigger for F16 and is usually resolved by cleaning or replacing the probe.
- Faulty or degraded ignition leads Common
The high-voltage leads connecting the PCB to the ignition electrode can crack, corrode at their terminals, or suffer insulation breakdown. A damaged lead prevents the spark from reaching the burner properly, causing ignition to fail before the flame sensor ever has a chance to detect anything.
- Insufficient gas reaching the burner Sometimes
If gas pressure at the boiler is lower than expected — whether due to a supply issue, a partially closed service valve, or a sluggish gas valve — the burner may light weakly or not at all. The flame sensor then fails to detect a stable flame and the boiler locks out.
- Faulty or sticking gas valve Sometimes
The gas valve controls the flow of gas to the burner. If internal components stick or the valve solenoid fails, gas may not flow freely even when the boiler calls for heat. This produces the same no-flame scenario that triggers F16.
- Fan or wiring fault causing overheating Rare
On some Glow-Worm models, a failing fan or damaged wiring can cause the boiler to overheat internally, prompting the control board to display F16 rather than a dedicated fan code. If a visual check of other components draws a blank, the fan and its associated wiring harness should be investigated.
- Faulty PCB Rare
The printed circuit board manages ignition sequencing and interprets the signal from the flame probe. A PCB fault can cause it to misread a good flame signal as absent, or fail to trigger the spark at the right moment. PCB faults are relatively uncommon but should be considered once other components have been ruled out.
How to fix it
- Check your gas supply is working DIY safe
Before anything else, confirm that other gas appliances in your home — a gas hob, for example — are working normally. If nothing gas-powered works, contact your gas supplier; the fault may be a supply interruption rather than a boiler problem.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Glow-Worm boiler for approximately 3–5 seconds, then release it. The boiler will run through its ignition sequence, which can take a few minutes. If it fires up and runs normally, monitor it closely over the next day or two. If F16 returns — or if the boiler fails to light — do not attempt further resets. Repeated forced ignition attempts can stress the PCB and burner components.
- Engineer to inspect and clean the ionisation probe Gas Safe engineer
A Gas Safe engineer will access the combustion chamber, remove the ionisation probe, and clean any carbon deposits from the tip using a fine abrasive pad or appropriate cleaner. This straightforward job often clears F16 on the spot and is the first thing a competent engineer will check.
- Engineer to test and replace ignition leads if necessary Gas Safe engineer
The engineer will check the ignition leads for continuity, cracks, and corrosion at both ends. Damaged leads are a common and inexpensive replacement. New leads restore the correct spark energy to the electrode, giving the burner the best chance of a clean light.
- Engineer to check gas pressure and inspect the gas valve Gas Safe engineer
The engineer will measure gas pressure at the boiler to confirm it falls within the manufacturer's specified range. If pressure is adequate but the valve is sluggish or failing, the valve will need to be replaced — a task that must only be carried out by a Gas Safe registered engineer.
- Engineer to test the fan and wiring harness Gas Safe engineer
If probe, leads, and gas supply are all found to be in good order, the engineer will turn attention to the fan motor and associated wiring. A fan running below its target speed or wiring with chafed insulation can produce abnormal control signals that result in F16 on certain models.
- Call a Gas Safe registered engineer if the fault persists or you have not already done so Gas Safe engineer
F16 will not resolve itself permanently without identifying the root cause. If you have already attempted a reset and the code has returned, contact a Gas Safe registered engineer to carry out a full diagnosis. You can verify any engineer's credentials at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- Ionisation / flame sensing probe · from £25
- Ignition electrode and lead set · from £45
- Gas valve · from £180
- Fan motor assembly · from £95
- 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 £90–£320, depending on the underlying cause.
Frequently asked questions
Can I reset my Glow-Worm boiler myself when F16 appears?
Yes, one reset attempt is reasonable while you wait to hear from an engineer. Hold the reset button for 3–5 seconds and give the boiler a few minutes to complete its ignition sequence. If it lights and stays on, keep an eye on it — F16 has a habit of returning if the underlying cause (usually a dirty probe or worn ignition lead) has not been fixed. Do not reset more than once or twice; persistent failed attempts can cause additional wear on the PCB and burner.
How much does it typically cost to fix a Glow-Worm F16 fault in the UK?
For the most common causes — cleaning or replacing the ionisation probe and ignition leads — you can expect to pay roughly £90 to £200 including labour. If the gas valve needs replacing, the total is more likely to fall in the £250–£320 range once parts and labour are combined. Less common components such as the PCB can push the cost higher still; if your engineer suspects the PCB is at fault, it is worth getting a quote and comparing it against the cost of a new boiler, especially if the appliance is over ten years old.
Why does F16 sometimes clear on its own and then come back?
A partially fouled ionisation probe or a slightly degraded ignition lead can still make intermittent contact with the flame or deliver just enough spark to light the burner — but not reliably. On a good day the boiler fires and the code disappears; on a bad day (or when components are cold and contracted) the connection fails and F16 reappears. Intermittent faults like this tend to get progressively worse, so a self-clearing F16 is still worth investigating sooner rather than later.
Is F16 the same as F27 on Glow-Worm boilers?
Both F16 and F27 fall into the flame detection category on Glow-Worm boilers, and they share many of the same possible causes — probe faults, ignition lead issues, and gas supply problems. The key difference is in exactly where in the ignition sequence the control board detects the problem, which can help an engineer narrow down the diagnosis. If your display shows F27 rather than F16, the troubleshooting approach is broadly similar, but your engineer will use the specific code to guide their component checks.