Glow-worm F27 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F27 fault code mean?
The F27 code indicates that your Glow-Worm boiler has attempted to ignite but its flame sensor cannot confirm a stable burner flame is present. As a safety measure, the boiler immediately locks out and shuts down all operation — both heating and hot water — to prevent any possibility of unburned gas accumulating. The boiler will not restart on its own; the underlying fault must be resolved before a reset will hold. On some Glow-Worm models, F27 appears alongside F16, and both relate to the same family of flame-detection faults — if your display is showing F16 rather than F27, the causes and remedies described here apply equally.
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 degraded flame sensor (ionisation rod) Common
The flame sensor works by detecting a tiny electrical current produced by the burner flame. Over time, carbon deposits and combustion residue build up on the sensor rod, insulating it and preventing an accurate reading. This is the single most common trigger for F27 on Glow-Worm boilers and is typically picked up during a service — cleaning or replacing the rod resolves it in most cases.
- Faulty or damaged ignition leads Common
The high-voltage leads that carry the spark to the burner electrodes can crack, corrode, or work loose over time, especially with repeated heat cycling. A poor connection means the spark either doesn't reach the burner consistently or doesn't generate enough energy to establish a reliable flame, causing the sensor to report no detection.
- Faulty gas valve Sometimes
The gas valve controls exactly how much gas is delivered to the burner. If it sticks, leaks internally, or fails to open fully, there may be insufficient gas for a flame to take hold even though the ignition spark is present. The boiler's sensor then correctly reports that no flame has been established. Gas valve work must only ever be carried out by a Gas Safe registered engineer.
- PCB (printed circuit board) fault Sometimes
The PCB orchestrates ignition timing, monitors the flame sensor signal, and decides whether a successful flame has been established. If moisture — even condensation — has reached the PCB, or if a component has failed, it may misread the sensor output and throw F27 even when combustion is occurring normally. PCB faults can also cause erratic ignition behaviour that prevents a stable flame from forming.
- Low system pressure or circulation issue Rare
Although less directly linked to flame detection, very low water pressure (below around 0.8 bar) or a struggling circulation pump can cause the boiler to overheat rapidly after ignition. Some models respond to this thermal stress by locking out with a flame-related code. Check your pressure gauge — a healthy reading is typically 1.0–1.5 bar when cold.
How to fix it
- Check your gas supply is live DIY safe
Before assuming a boiler fault, confirm that your gas supply is working. Check whether other gas appliances in the property — a hob or gas fire — are operating normally. If no gas appliances are working, contact your gas supplier; there may be a supply interruption in your area. Do not attempt to work on any gas pipework yourself.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler (usually on the front panel or underneath the boiler). If it reads below 1 bar, low pressure may be contributing to lockout behaviour. You can top up system pressure yourself using the filling loop — consult your boiler's user guide for the location of the filling loop valves and follow the manufacturer's instructions to bring pressure to around 1.2–1.5 bar. Do not overpressure the system beyond 2 bar.
- Reset the boiler DIY safe
Once you have confirmed gas is available and pressure is adequate, attempt a reset. On most Glow-Worm models this involves turning the central heating control knob to the reset position (marked with a power or reset symbol), holding it briefly, then releasing and setting to your desired temperature. Allow the boiler a full ignition attempt — typically 30–60 seconds. If the boiler fires up and runs normally, monitor it for the next few hours. If F27 returns, do not keep resetting repeatedly; two or three attempts maximum is sufficient before calling an engineer.
- Do not attempt to clean, adjust, or replace the flame sensor yourself Gas Safe engineer
Although cleaning the ionisation rod sounds straightforward, accessing it requires partially dismantling the boiler casing and working in close proximity to the gas train and ignition components. This work must be carried out by a Gas Safe registered engineer who can also test the sensor's output with the appropriate diagnostic equipment and replace it safely if needed.
- Do not attempt to inspect or replace ignition leads, the gas valve, or the PCB yourself Gas Safe engineer
All of these components sit inside the boiler casing and involve either gas connections or high-voltage electrical circuits. Incorrect handling creates serious risks of gas leaks, electric shock, or an uncontrolled ignition event. A Gas Safe registered engineer will test each component in a structured sequence — sensor, leads, valve, then PCB — to find the root cause before replacing anything unnecessarily.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has locked out on F27 and a reset has not resolved it, you need a qualified engineer. Provide them with the fault code and describe how many times the boiler has locked out and over what period — this helps narrow down whether the fault is intermittent (more suggestive of a sensor or lead issue) or persistent (more suggestive of a gas valve or PCB problem). Always verify the engineer is Gas Safe registered at gassaferegister.co.uk before booking.
Parts you may need
- Flame sensor (ionisation rod) · from £25
- Ignition leads (pair) · from £35
- Ignition electrode assembly · from £40
- Gas valve · from £130
- PCB (main 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F27 fault myself?
The initial checks — confirming your gas is on, topping up system pressure if it is low, and attempting a reset — are safe for a homeowner to carry out. Beyond that, the fault involves internal boiler components: the flame sensor, ignition leads, gas valve, or PCB. Working on any of these requires Gas Safe registration. Attempting it yourself is both dangerous and illegal, and could invalidate your boiler warranty or home insurance.
How much will it cost to fix a Glow-Worm F27 fault?
For the most common repairs — cleaning or replacing the flame sensor, or renewing the ignition leads — you can typically expect to pay in the region of £120 to £250 including the engineer's call-out and labour. If the gas valve needs replacing, costs usually rise to around £250–£380. A PCB replacement is less common but more expensive; it can run to £400–£550 depending on the model and parts availability. Most F27 faults turn out to be a sensor or lead issue, so the majority of customers pay in the lower part of this range.
Why does my Glow-Worm boiler keep coming back with F27 after a reset?
A reset clears the lockout but does not fix the underlying fault. If F27 returns after one or two resets, the boiler's ignition system has a repeatable problem — most often a contaminated flame sensor or a failing ignition lead — that will not resolve itself. Continuing to reset the boiler without a proper diagnosis wastes time, can mask a worsening fault, and in the case of a gas valve issue carries a safety risk. Book a Gas Safe engineer to investigate as soon as possible.
Is F27 the same as F16 on a Glow-Worm boiler?
Both F16 and F27 relate to flame detection on Glow-Worm boilers and share most of the same root causes. F27 specifically indicates the boiler locked out after failing to confirm flame presence during the ignition sequence, while F16 can also appear during normal operation if the flame signal is lost unexpectedly. The diagnostic process and the components checked are essentially the same for both codes, so the information on this page applies if you are seeing either one.