Glow Worm F1 Fault Code: Ignition Lockout Explained
The F1 code appears on Glow Worm boilers when the boiler has attempted to light itself five times in a row without the PCB detecting a stable flame. After these repeated failed attempts, the boiler locks out completely — cutting off both heating and hot water — as a built-in safety measure to prevent dangerous gas accumulation or incomplete combustion. Because the boiler's display doesn't pinpoint which specific stage of the ignition sequence failed, diagnosing the root cause requires a methodical process of elimination.
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.
What does the Glow-worm F1 fault code mean?
The F1 code appears on Glow Worm boilers when the boiler has attempted to light itself five times in a row without the PCB detecting a stable flame. After these repeated failed attempts, the boiler locks out completely — cutting off both heating and hot water — as a built-in safety measure to prevent dangerous gas accumulation or incomplete combustion. Because the boiler's display doesn't pinpoint which specific stage of the ignition sequence failed, diagnosing the root cause requires a methodical process of elimination.
Common causes
- Frozen or blocked condensate pipe Common
In cold weather, the external section of the condensate pipe — which drains waste water from the boiler — can freeze solid. When this happens the boiler shuts itself down to avoid a blockage backing up internally. This is by far the most frequent cause of F1 lockouts during winter months, and it is the one cause a homeowner can safely resolve without an engineer.
- Faulty ignition electrode or spark probe Common
The spark electrode generates the ignition spark that lights the gas. Over time electrodes can crack, accumulate carbon deposits, or simply wear out, meaning no reliable spark is produced. The ignition lead connecting the electrode to the PCB can also degrade, interrupting the electrical signal. Because the boiler can't confirm ignition, it codes F1.
- Gas supply disruption Common
If the gas supply to the property is interrupted — whether due to a closed isolation valve, an unpaid bill, or a wider supply issue in your street — the boiler will attempt to fire but find nothing to burn. Checking whether other gas appliances such as a hob or gas fire are working normally is a quick way to rule this out.
- Dirty or faulty flame sensor Common
A separate component from the ignition electrode, the flame sensor (also called the ionisation probe) checks that a flame is actually present once ignition has been attempted. If it is coated in carbon deposits or has become faulty, it may fail to detect a healthy flame even when one exists, causing the boiler to assume ignition has failed and lock out.
- Low system pressure Sometimes
Glow Worm boilers are designed to operate between roughly 1 and 1.5 bar when cold. If system pressure drops below this range — often due to a small leak or a recently bled radiator — the boiler may refuse to complete its ignition sequence and display F1 alongside or instead of a pressure-specific code.
- Blocked or stuck gas valve Sometimes
The gas valve controls precisely how much gas enters the burner during ignition and normal operation. If it becomes stuck, seized, or fails electrically, gas cannot reach the burner in the correct quantity, so ignition cannot occur. Gas valve faults tend to be more common on older boilers or those that have not been serviced regularly.
- Blocked burner jets Sometimes
Small jets inside the burner mix gas and air to produce the flame. Carbon deposits or debris can partially or fully block these jets, meaning insufficient gas reaches the burner for a stable flame to establish. An engineer can remove and clean blocked jets without necessarily replacing the whole burner assembly.
- Blocked or restricted flue Sometimes
The flue expels combustion gases safely outside. A blockage — from a bird nest, debris, or ice build-up — can restrict airflow to the point where the boiler's fan cannot establish the correct pressure differential, preventing ignition from being permitted. The boiler locks out as a safety precaution.
- Fan failure or poor fan performance Sometimes
The combustion fan runs before ignition to purge the combustion chamber and establish the correct airflow. If the fan is failing, running slowly, or has seized, the boiler's air-pressure sensor will not confirm that conditions are safe to ignite, and the F1 lockout follows.
- PCB fault Rare
The printed circuit board orchestrates the entire ignition sequence — sending signals to the fan, gas valve, ignition electrode, and reading back from the flame sensor. A failing or damaged PCB can send incorrect signals or misread sensor data, producing an F1 code even when all the physical components are in good condition. PCB faults are relatively uncommon but tend to be the most expensive to resolve.
How to fix it
- Check that your gas supply is working DIY safe
Before anything else, confirm that gas is reaching your home. Try another gas appliance — a hob burner or gas fire — to see if it lights normally. If nothing gas-powered is working, the problem lies with your supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency line on 0800 111 999 if you suspect a supply fault or smell gas.
- Check and top up the system pressure DIY safe
Look at the pressure gauge on the front of the boiler. If it reads below 1 bar, low pressure may be contributing to the lockout. Locate the filling loop — usually a silver or grey braided hose with one or two valves beneath the boiler — and slowly open the valve(s) until the gauge reads between 1 and 1.5 bar, then close them firmly. If the pressure drops again quickly, there may be a leak that an engineer should investigate.
- Check for a frozen condensate pipe (cold weather only) DIY safe
If temperatures have recently dropped below freezing, inspect the condensate pipe — typically a white plastic pipe, 21.5 mm or 32 mm in diameter, exiting through an outside wall and running to a drain. If it feels solid or you can see ice, pour warm (not boiling) water along the frozen section using a watering can or a cloth soaked in warm water. Work from the boiler end towards the drain outlet. Once thawed, reset the boiler and monitor it. Lagging the pipe with foam pipe insulation afterwards will reduce the risk of it happening again.
- Reset the boiler DIY safe
Once you have completed the checks above, press and hold the reset button (often marked with a flame symbol or labelled 'Reset') for around three seconds. Release it and allow the boiler up to two minutes to run through its start-up sequence. If the F1 code clears and the boiler fires up, keep an eye on it over the next hour to make sure it stays running. If it locks out again, attempt the reset no more than two or three times in total — repeated resets without addressing the underlying fault can mask a more serious problem.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler continues to display F1 after the checks above, the fault is most likely internal — a worn ignition electrode, a faulty flame sensor, a gas valve problem, a fan issue, or in rarer cases a PCB fault. All of these require a qualified Gas Safe registered engineer to diagnose and repair safely. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk. Working on gas components without Gas Safe registration is illegal and potentially life-threatening.
Parts you may need
- Ignition electrode / spark probe · from £35
- Ignition lead · from £25
- Flame sensor (ionisation probe) · from £30
- Gas valve · from £180
- Combustion fan · from £120
- 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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to get the heating back on?
You can try resetting up to two or three times, but it is not advisable to keep repeatedly resetting a boiler showing F1. The lockout exists for a safety reason — the boiler could not confirm that ignition occurred correctly. Continually resetting without fixing the underlying problem risks allowing unburned gas to accumulate, which is dangerous. If two or three resets don't clear the fault, call an engineer.
Is the Glow Worm F1 code always a gas-related problem?
Not necessarily. A frozen condensate pipe, which any homeowner can thaw themselves, is the single most common cause of F1 lockouts in winter and has nothing to do with the gas components. Low system pressure is another non-gas trigger you can check yourself. That said, if the simple DIY checks don't resolve it, the fault is very likely in the ignition or gas-delivery system and will need a Gas Safe engineer.
How much does it cost to fix a Glow Worm F1 fault?
For the most common causes — a thawed condensate pipe or a top-up of system pressure — there is no parts cost, just your time. When an engineer is needed, straightforward fixes such as replacing an ignition electrode, spark lead, or flame sensor typically cost £100–£180 including labour. A gas valve replacement usually runs to £280–£380 all in. In rare cases where the PCB is at fault, costs can exceed £400–£500; at that point, on an older boiler, a full replacement is often worth considering.
My boiler is making a clicking sound but not lighting — is that definitely F1?
Almost certainly yes. That clicking is the spark electrode attempting to ignite the gas. If the boiler clicks repeatedly and then stops without firing, it has failed to detect a flame and will lock out, displaying F1. The clicking itself tells you the ignition system is trying to work, which actually narrows the fault — it is more likely a gas supply issue, a faulty flame sensor, or insufficient gas reaching the burner than a completely dead electrode.