Glow-worm boiler fault codes — what they mean and what to do
Seeing an F code flash up on your Glow-worm? Here's what it's telling you, what's safe to check yourself, and when to call us out.
What a fault code is telling you
Your Glow-worm boiler keeps an eye on itself. When something isn't right, it stops and shows a code on the display so you know roughly what's wrong.
Most codes look alarming but aren't. Some point to simple things you can sort in a few minutes, like low pressure or a frozen condensate pipe in cold weather.
Others mean a part has failed and needs a Gas Safe registered engineer. We'll always tell you which is which, and never ask you to touch anything to do with gas.
Glow-worm codes nearly always start with "F". Wording varies a little between Energy, Easicom, Ultimate, Betacom and older models, so check your manual for the exact text. The most common ones are below.
Common Glow-worm fault codes
| Fault code | What it means | What to do |
|---|---|---|
| F.22 | Low water pressure. The system has dropped too low for the boiler to fire. | Re-pressurise to about 1.0–1.5 bar using the filling loop. If it keeps dropping you've likely got a leak — book a Gas Safe registered engineer. |
| F.9 | Water pressure sensor fault (on some models a blocked flow filter or pump fault). This is about the sensor or flow, not simply low pressure. | Check the pressure gauge. If it reads a normal 1.0–1.5 bar, this is a sensor or flow fault — book a Gas Safe registered engineer rather than repeatedly topping up. |
| F.28 / F.29 | Ignition has failed — the boiler tried to light and couldn't. Often a gas supply issue (including a pre-payment meter out of credit) or a frozen/blocked condensate pipe. Older Glow-worm models use F.1 or F.4 for ignition/flame-detection faults. | Check your other gas appliances work and that any pre-payment meter is in credit. In cold weather, check for a frozen condensate pipe. Reset once. If it returns, book an engineer. Smell gas? Call 0800 111 999. |
| F.1 / F.4 | Ignition or flame-detection fault on older Glow-worm appliances (newer models use F.28/F.29). In cold weather this can also point to a frozen condensate pipe. | Check the condensate pipe isn't frozen and other gas appliances work. Reset once. If it sticks, book a Gas Safe registered engineer. |
| F.75 | The boiler couldn't detect a pressure rise when it ran the pump on start-up. Usually the pressure sensor is blocked with sludge, or the pump has failed. | Check pressure is at 1.0–1.5 bar and reset once. If F.75 returns, the sensor or pump needs an engineer — not a DIY fix. |
| F.73 / F.74 | Water pressure sensor fault — the sensor reading is too low, too high, or out of range. | Check the pressure gauge. This is a sensor fault, so book a Gas Safe registered engineer to test and replace it. |
| F.5 | Overheat fault — the safety cut-out has tripped because the boiler got too hot. | Don't keep resetting it. Check pressure and that radiator valves are open, then book a Gas Safe registered engineer. An overheat lockout is a genuine safety trip. |
| F.20 | Overheating fault — a safety temperature limit has been exceeded. | Check pressure and for trapped air or sludge. Don't repeatedly reset an overheat fault — book an engineer to check the heat exchanger and pump. |
| F.27 | Flame detection fault — the boiler senses a flame when there shouldn't be one (a false-flame signal). | This involves the flame circuit and gas valve, so it's not a DIY fix. Reset once, then book a Gas Safe registered engineer. Smell gas? Call 0800 111 999. |
| F.23 | Circulation fault — temperature difference too great, so water isn't moving through the boiler properly (often a stuck pump or airlock). | Check pressure and that radiator valves are open. If it persists, book an engineer to look at the pump and circulation. |
| F.25 | Flue gas temperature too high — a safety switch-off. | This is a safety trip. Don't keep resetting it — book a Gas Safe registered engineer to investigate. |
| F.6 / F.7 / F.10 | Temperature sensor (thermistor) fault on the heating flow, hot water or return. | You may still have some heating. This needs a sensor test and replacement by a Gas Safe registered engineer. |
| F.61–F.65 | Gas valve / control electronics fault. | This is an internal electronics fault. Book a Gas Safe registered engineer — there's nothing safe to try yourself. |
| Err | User interface fault — the display or its connection to the main board isn't communicating. | Turn the boiler off and on once. If "Err" stays, book a Gas Safe registered engineer. |
Safe checks before you call anyone out
These are the only things worth trying yourself. If a code involves gas, skip straight to a Gas Safe registered engineer.
- Check the pressure gauge — it should sit around 1.0 to 1.5 bar when cold.
- Top up via the filling loop if pressure's low, then close the valves fully.
- In cold weather, check the external condensate pipe isn't frozen, and thaw it gently with warm (not boiling) water if it's safe to reach.
- Make sure the boiler has power and the programmer is set to come on.
- Check your thermostat is calling for heat and the batteries aren't flat.
- If you're on a pre-payment gas meter, check it's in credit, and confirm your other gas appliances work.
- Reset the boiler once — just once. Don't keep doing it if the code returns, and don't repeatedly reset an overheat fault (F.5 or F.20).
- If you smell gas, stop everything. Call 0800 111 999, open windows, turn off the gas at the meter, don't touch electrical switches, and leave the house.
The codes we get asked about most
F.22 — low pressure
This is the friendliest fault to see, and the most common. Your system pressure has dropped too low for the boiler to fire.
Find the filling loop — usually a silver braided hose under the boiler. Open both valves slowly until the gauge reads between 1.0 and 1.5 bar, then close them fully.
If pressure keeps falling after a top-up, you've probably got a small leak somewhere. We'll come and fix it for you.
Don't confuse F.22 with F.9. F.9 points to a water pressure sensor fault (or, on some models, a blocked flow filter or pump fault) rather than simply low pressure — so if your gauge already reads a normal 1.0–1.5 bar, repeatedly topping up won't help and you'll need a Gas Safe registered engineer.
F.28 and F.29 — ignition failure
These mean the boiler tried to light and couldn't find a flame, so it shut off to stay safe. Older Glow-worm models use F.1 or F.4 for ignition and flame-detection faults; newer models use F.28/F.29.
First, check your other gas appliances. If a pre-payment meter has run out of credit, top it up. In cold weather, check the external condensate pipe hasn't frozen, as that blocks the boiler too. Reset once.
If F.28 comes straight back, stop there. The ignition or gas valve may need a Gas Safe registered engineer. If you smell gas at any point, call 0800 111 999.
F.75 — pump or pressure sensor
F.75 means the boiler couldn't detect a rise in pressure when it ran the pump during its start-up checks. It's one of the more common Glow-worm faults.
It's usually caused by the pressure sensor being blocked with magnetite sludge, or by a failed pump. You can check the pressure is correct and reset once, but if F.75 returns the sensor or pump needs replacing — that's an engineer job, not a DIY fix.
When a code won't clear — how Smart Plan helps
If you've checked the simple stuff and the code's still there, the next step is a Gas Safe registered engineer. That's where we come in.
Smart Plan is a service plan, not insurance. You've got two easy options when your boiler plays up.
Need it fixed now?
Book a one-off repair and we'll come and fix it for you. One of our Gas Safe registered engineers diagnoses the fault and gets your heating back on.
Want cover for next time?
Take out an ongoing Smart Plan boiler module and you're sorted when the next code appears. Parts and labour are included up to your cover limit.
Cover is modular, so you only pick what you want — don't pay for what you don't use. Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies.
We've looked after over 15,000 customers, we've been trading since 2014, and we're rated Excellent on Trustpilot. When a fault code won't budge, we'll come and fix it for you.
Glow-worm fault code FAQs
Is boiler cover the same as insurance?
No — it's a service plan, not insurance. You pick the cover modules you want, and when something breaks we send a Gas Safe registered engineer to fix it, with parts and labour included up to your cover limit.
What does F.22 mean on a Glow-worm boiler?
F.22 means low water pressure — the system has dropped too low for the boiler to fire. Re-pressurise to about 1.0–1.5 bar using the filling loop, then close the valves fully. If pressure keeps dropping you've likely got a leak, so book a Gas Safe registered engineer. Don't confuse it with F.9, which is a pressure sensor fault rather than simply low pressure.
What does F.75 mean on a Glow-worm boiler?
F.75 means the boiler couldn't detect a rise in pressure when it ran the pump on start-up. It's usually a pressure sensor blocked with sludge or a failed pump. Check the pressure and reset once. If it returns, the sensor or pump needs a Gas Safe registered engineer — it isn't a safe DIY fix.
How many times should I reset my boiler?
Just once, and not repeatedly for an overheat fault like F.5 or F.20. If the same code comes straight back after a reset, stop. Repeated resets can hide a real fault. Book a Gas Safe registered engineer instead.
Can I fix a Glow-worm fault code myself?
Only the safe basics — topping up pressure, thawing a frozen condensate pipe, checking the thermostat, or a single reset. Anything involving gas, the pump, the gas valve or internal parts needs a Gas Safe registered engineer. Never attempt a gas repair yourself.
Why does my Glow-worm boiler show F.28 in cold weather?
F.28 means ignition failed (older models use F.1 or F.4 for ignition/flame-detection faults). In cold weather this is often a frozen condensate pipe. Check for a frozen external pipe and thaw it gently with warm (not boiling) water if it's safe to reach, and make sure any pre-payment gas meter is in credit. Reset once. If it returns, book a Gas Safe registered engineer.
What should I do if I smell gas?
Call the National Gas Emergency Service on 0800 111 999 straight away. Open your windows, turn off the gas at the meter, don't touch any electrical switches, and leave the property.
Does Smart Plan cover Glow-worm boilers?
Yes. The Smart Plan boiler module is a service plan that covers boiler repairs up to your cover limit — up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older, with a £95 call-out fee. A Gas Safe registered engineer carries out the work.
Code still showing? We'll come and fix it for you.
Book a one-off repair or set up an ongoing Smart Plan boiler module. A service plan, not insurance — parts and labour included up to your cover limit.
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