Your Glow-worm boiler shows a fault code when something has gone wrong and it has shut down to protect itself. The code — usually an F followed by a number — tells you roughly what triggered the lockout. F22 commonly relates to low water pressure, F75 to pressure detection or circulation, and F1 to an ignition fault. Some faults can be cleared with a safe external check. Others need a Gas Safe engineer.
This page covers the common Glow-worm fault codes we get called out for, what they can mean, the safe checks you can do first, and when to stop and call someone. Fault-code meanings can vary by boiler range, so use the model-specific manual where a code differs. If you already know your code, jump to it below. If your boiler has died with no code showing at all, we cover that too.
We're a London reactive maintenance company. We repair Glow-worm boilers across the city every week, so the advice here is what we'd actually tell you on the phone — not filler.
- Content Guide
- » What a Glow-worm Fault Code Actually Is
- » F22 — Low Water Pressure
- » F1 — Ignition Fault
- » F4 — Ignition Fault
- » F5 — Overheat Fault
- » F9 — Pressure Sensor / Circulation Fault
- » F13 — PCB Memory / Sensing Fault
- » F14 — Gas Valve Control Fault
- » F28 — Ignition Failure
- » F29 — Flame Loss During Operation
- » F75 — Pump or Pressure Sensor Fault
- » F83 — Water Temperature Fault
- » Flexicom 30cx Fault Codes
- » 15hxi Faults
- » Glow-worm Boiler Not Working With No Fault Code
- » Which Faults You Can Fix Yourself
- » When to Call an Engineer
- » Get Your Glow-worm Boiler Fixed
What a Glow-worm Fault Code Actually Is
Glow-worm boilers have a built-in diagnostic system. When a sensor reads something outside its safe range — pressure too low, flame not detected, a component overheating — the boiler locks out and shows a code on its display.
The code isn't the problem itself. It's the boiler's best guess at the cause. F22 doesn't mean the pressure gauge is broken, it means it has detected low pressure. The job is working out why the pressure is low.
Most codes fall into a few buckets: pressure faults, ignition and gas faults, sensor faults, and circulation faults. Glow-worm has reused some code numbers across different boiler ranges, so the model and manual matter as much as the code itself. The summaries below are aligned to the linked supporting pages and manufacturer fault tables, with model differences called out where needed.
A quick word on safety. Never remove the boiler casing. Never touch gas components. If you smell gas, turn off the supply at the meter, open windows, and call the National Gas Emergency line on 0800 111 999. Everything below is either a display check or something on the outside of the unit.
F22 — Low Water Pressure
F22 is the code we see most. It means the water pressure in your central heating system has dropped below the level the boiler needs to run.
Check the pressure gauge on the front of the boiler. It should sit between 1 and 1.5 bar when cold. If it's reading 0.5 or lower, that's your fault. You can usually top it up using the filling loop — the small silver braided hose underneath — but if the pressure keeps dropping every few weeks, you've got a leak somewhere and topping up is just masking it.
For the full breakdown of causes, symptoms, and step-by-step re-pressurising, read our guide on how to fix the Glow-worm F22 fault code
F1 — Ignition Fault
On common Glow-worm ranges such as Flexicom, Ultracom and cxi, F1 is an ignition fault. A gas-supply interruption or low gas pressure can be one cause, but ignition components, the gas valve, wiring or other internal faults may also prevent the boiler establishing a flame.
If gas is definitely on and reaching other appliances, the fault is inside the boiler and needs an engineer. Don't keep resetting it repeatedly.
Full detail is in our guide on the Glow-worm F1 fault
F4 — Ignition Fault
On common Flexicom, Ultracom, cxi and SI ranges, F4 is an ignition fault. It means the boiler has not completed the ignition sequence successfully; the exact cause can involve gas supply, ignition or flame-proving components, wiring or controls. This is different from F29, where a flame has been established and is then lost during operation.
This isn't a homeowner repair. A Gas Safe engineer may need to check ignition, flame-proving, gas-valve, wiring or control components. Our guide on the Glow-worm F4 fault explains what's involved.
F5 — Overheat Fault
On common Glow-worm Flexicom, Ultracom, cxi and SI ranges, F5 is an overheat fault. Restricted circulation, a pump problem, low system pressure, temperature-sensing issues or another internal fault can contribute, and the boiler locks out to protect itself.
If the fault persists, an engineer should diagnose the circulation, pump, temperature-sensing and other internal causes. See our guide on the Glow-worm F5 fault
F9 — Pressure Sensor / Circulation Fault
On common Flexicom and Ultracom ranges, F9 points to a water-pressure sensor or pressure-detection fault and can also be associated with a blocked flow filter or pump/circulation issue. It should not be presented as a general communication error.
A reset sometimes clears it if it was a one-off glitch, but a recurring F9 needs looking at. Read our guide on the F9 fault on a Glow-worm boiler.
F13 — PCB Memory / Sensing Fault
On common Flexicom and Ultracom ranges, F13 is associated with PCB memory, sensing or connection faults. Diagnosis may involve the PCB, connected sensors, wiring harnesses or electrical connections, so internal testing should be left to a qualified engineer.
Our guide on the Glow-worm F13 fault covers the likely causes and what an engineer will check.
F14 — Gas Valve Control Fault
On Flexicom, Ultracom and some related Glow-worm ranges, F14 is a gas-valve control fault. Some older cxi/SI ranges use F14 for excessive central-heating flow temperature, so the boiler model should be checked before diagnosis. This supporting page focuses on the gas-valve-control meaning.
If a single reset doesn't clear it and hold, you need an engineer. See our guide on how to fix a Glow-worm F14 fault.
F28 — Ignition Failure
F28 means the boiler tried to fire up and failed to ignite. It's one of the more common lockouts alongside F22. Causes range from no gas supply, to a faulty ignition lead, to a condensate blockage in cold weather.
The condensate pipe is worth a quick check yourself — if it froze overnight, thawing it can clear the fault. Beyond that, it's an engineer job. Full detail in our guide on the Glow-worm F28 fault code.
F29 — Flame Loss During Operation
F29 is close to F28 but slightly different: the flame lit and then went out during operation. The boiler detects the flame has disappeared and locks out.
Like F28, a frozen condensate pipe or a gas supply interruption can be behind it. Our guide on the Glow-worm F29 fault walks through the causes and fixes.
F75 — Pump or Pressure Sensor Fault
F75 means the boiler isn't detecting a pressure change when the pump starts. That usually points to a faulty pump, a stuck pressure sensor, or air trapped in the system.
It's a frustrating one because it can look like a pressure fault but the gauge reads fine. Diagnosis matters here. Read our guide on the Glow-worm F75 fault code.
F83 — Water Temperature Fault
F83 means the boiler has detected no temperature rise, or too small a rise, at the flow and return after the burner starts. Depending on the model, insufficient water/circulation or incorrectly reading or positioned flow/return sensors may be involved.
A safe first check is the pressure shown on the boiler, following the model manual. Sensor, wiring and circulation testing is engineer-only. Full detail is in our guide on how to fix the F83 Glow-worm fault
Flexicom 30cx Fault Codes
The Flexicom range has its own quirks, and the 30cx in particular throws codes that owners search for by model rather than by number. If you've got a Flexicom, it's worth reading the model-specific guidance rather than the generic code explanations.
We've put together a full page on Glow-worm Flexicom 30cx fault codes covering the codes specific to that unit.
15hxi Faults
The 15hxi is an older Glow-worm unit and it has its own common failure points. If yours is playing up — whether it's showing a code or just behaving oddly — the model-specific guide is more useful than the general list.
See our guide on Glow-worm 15hxi faults for the issues we see most on that boiler.
Glow-worm Boiler Not Working With No Fault Code
Sometimes the boiler just stops — no heating, no hot water, and no code on the display at all. That's often more worrying than a coded fault, because there's nothing telling you where to start.
The usual suspects are power, the thermostat (dead batteries stop the boiler firing), a tripped RCD, or the timer settings. Our guide on a Glow-worm boiler not working with no fault code runs through the checks in order.
Which Faults You Can Fix Yourself
Be honest about what's safe. You can:
- Re-pressurise the system for F22 using the filling loop
- Check the gas supply is on for F1, F28 and F29
- Thaw a frozen condensate pipe for F28 and F29 in cold weather
- Check power, thermostat batteries and the programmer when there's no code
You should not attempt anything that involves removing the casing or touching gas or electrical components inside the boiler. That includes internal ignition, circulation, pressure-sensing, PCB and gas-control diagnosis for faults such as F4, F5, F9, F13 and F14, as well as recurring lockouts. Gas work and internal boiler diagnosis should be left to a suitably qualified Gas Safe engineer.
When to Call an Engineer
Call someone if any of these apply:
- The same fault code keeps coming back after a reset
- You've topped up the pressure and it drops again within days
- There's no code but the boiler won't run
- You smell gas (turn off the supply and call 0800 111 999 first)
- You see water leaking from or around the boiler
- The boiler is making banging or gurgling noises
There's no prize for struggling on with cold radiators. If it's beyond a display check, get a professional boiler repair in London booked and sorted properly.
Get Your Glow-worm Boiler Fixed
If you've worked through the checks and your Glow-worm is still faulting — or you'd rather not risk it — we can help. Our Gas Safe engineers repair Glow-worm boilers across London, with appointment availability depending on engineer capacity and your location.
We'll diagnose the actual cause, not just clear the code, so it doesn't come straight back. Book a boiler repair in London and we'll get your heating running again.
