Glow-worm F3 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F3 fault code mean?
The F3 code means your Glow-Worm boiler has detected that its fan is not spinning fast enough to reach the speed required for safe ignition. Before the burner is allowed to fire, the boiler's controls verify that the fan is moving at the correct rate to safely carry combustion gases out through the flue. If that target speed isn't achieved within the allowed time, the boiler locks out and displays F3 as a protective measure. Because the fan is responsible for expelling potentially harmful gases — including carbon monoxide — this lockout is a deliberate safety response, not a nuisance trip.
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
- Worn or failing fan motor Common
Over time, the motor bearings in the fan wear down or the motor windings deteriorate, preventing the fan from reaching full speed. This is by far the most frequent root cause of an F3 fault. You may hear a sluggish whirring or grinding noise just before the boiler locks out.
- Loose or damaged wiring to the fan Common
The electrical connections between the PCB and the fan can work loose through vibration or general wear, causing intermittent or incorrect speed signals. The boiler may lock out randomly rather than every time if a connector is only partially loose.
- Flue or air-inlet blockage Sometimes
A partial obstruction in the flue or the air-intake duct — such as a bird's nest, debris, or a kinked flexible flue liner — creates back-pressure that makes it harder for the fan to move air at the required rate, triggering an F3.
- Frozen or blocked condensate pipe Sometimes
In cold weather, a frozen condensate pipe can cause a water build-up that restricts airflow through the system, indirectly affecting the fan's ability to reach ignition speed. This cause is seasonal and usually presents alongside other symptoms such as a gurgling sound.
- Air pressure switch fault Sometimes
The air pressure switch monitors whether the fan has created sufficient draught. If the switch itself is faulty or its sensing tube is blocked or split, it can send incorrect signals to the PCB — making the boiler believe the fan speed is inadequate even when the fan is working normally.
- PCB fault Rare
In rarer cases the printed circuit board itself is either misreading the fan speed sensor or failing to send the correct drive signal to the fan motor. PCB faults tend to occur only after other causes have been ruled out.
How to fix it
- Switch the boiler off and check for any obvious warning signs DIY safe
Turn the boiler off at the programmer or room thermostat. Take a moment to note whether you can smell gas or see any water leaks. If you smell gas, do not attempt any checks — turn off the gas supply at the meter, open windows, and call the National Gas Emergency Service on 0800 111 999 immediately.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (such as a gas hob) are working normally. If they are not, the issue may be with your gas supply rather than the boiler — contact your gas supplier.
- Inspect the condensate pipe for freezing (winter months only) DIY safe
If the weather is below freezing, go outside and locate the condensate pipe — typically a white plastic pipe (usually 21.5 mm or 32 mm) that exits through an external wall. If it feels solid or you can see ice, thaw it gently with warm (not boiling) water or a hot-water bottle wrapped around the pipe. Never use a naked flame.
- Reset the boiler and observe carefully DIY safe
Once you have checked the above, attempt a boiler reset using the reset button on the front panel — refer to your manual if you are unsure of its location. Watch and listen as it tries to start: a struggling, slow, or noisy fan is a strong indicator that the fan motor itself is the problem. Limit resets to two or three attempts; repeated unsuccessful resets can cause additional wear.
- Check the flue terminal outside for obvious blockages DIY safe
If it is safe to do so, visually inspect the flue outlet on the outside wall or roof. Look for any visible debris, nesting material, or ice covering the terminal. Do not attempt to dismantle the flue yourself — if you see a blockage you cannot remove from the outside without tools, leave it for an engineer.
- Contact a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler continues to display F3 after the above checks, the repair will require a qualified professional. An engineer will test the fan motor, measure its running speed, inspect the wiring harness and connectors, check the air pressure switch and its sensing tube, and examine the flue for blockages. Depending on findings, they may replace the fan motor alone, the entire fan assembly, the air pressure switch, or in more unusual circumstances the PCB.
Parts you may need
- Boiler fan / fan assembly · from £120
- Fan motor (motor only, where serviceable) · from £65
- Air pressure switch · from £35
- Fan wiring harness / connector · from £20
- 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 £220–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F3 fault myself?
Homeowners can carry out a small number of checks safely — verifying the gas supply is on, thawing a frozen condensate pipe, and attempting a reset. Beyond that, the F3 fault requires a Gas Safe registered engineer. Working on the fan, wiring, or any internal boiler component without the appropriate registration is illegal for gas appliances in the UK and could be dangerous.
How much does it cost to repair a Glow-Worm F3 fault in the UK?
For most households the repair bill falls between around £220 and £380 including parts and labour. A straightforward fan motor swap tends to sit towards the lower end of that range, while a full fan assembly replacement with a higher call-out rate (particularly in London and the South East) will push costs higher. A PCB replacement — needed only in a minority of F3 cases — can add significantly more; if your engineer suspects the PCB is at fault, it is worth getting a quote before committing, as on an older boiler a new boiler may sometimes make better financial sense.
Why does my Glow-Worm boiler keep showing F3 after I reset it?
If F3 returns repeatedly after resets, the underlying fault has not gone away. The most likely culprits are a fan motor that is deteriorating and losing speed more quickly each time, or a loose wiring connection that is intermittently breaking contact. Continuing to reset without a repair risks further wear on the fan and other components — it is worth getting an engineer out sooner rather than later.
Is it safe to use my boiler if it shows an F3 fault?
No. F3 is a safety lockout specifically because the fan — which removes combustion gases from your home — is not functioning correctly. Running the boiler with a compromised fan could lead to a build-up of carbon monoxide, which is odourless and potentially fatal. Leave the boiler switched off until a Gas Safe engineer has investigated and resolved the fault. If you have a carbon monoxide alarm and it is sounding, evacuate immediately and call 999.