Glow-worm F9 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F9 fault code mean?
The F9 fault code on a Glow-Worm boiler indicates that the boiler's control board has detected a problem with system water pressure or with the pressure sensor itself. This could mean pressure has dropped below the safe operating threshold (typically around 1 bar), risen too high (above 2 bar), or that the sensor responsible for monitoring pressure is delivering faulty or no readings to the PCB. In any of these scenarios, the boiler locks out as a safety precaution, stopping both heating and hot water until the underlying issue is addressed. You may also notice lukewarm radiators, unexpected boiler shutdowns, fluctuating hot water temperatures on a combi, or unusual gurgling and banging noises from the pipework.
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
- Low system pressure Common
The single most common trigger for F9. System pressure naturally drops over time through micro-leaks at radiator valves, pipe joints, or the boiler itself, falling below the 1 bar minimum the boiler needs to operate safely. A reading below 0.8–1 bar on the pressure gauge almost always explains an F9 lockout.
- Recent radiator bleeding Common
Bleeding radiators to remove air is good maintenance, but it also releases water from the system. If pressure wasn't topped up afterwards, it can drop enough to trigger F9 within hours or days.
- Faulty or blocked pressure sensor Sometimes
The pressure sensor can become coated in system sludge and debris, preventing it from measuring accurately. Even with correct system pressure, a sensor reading falsely low or high will still cause the PCB to log an F9 fault. The sensor may need cleaning or replacement.
- High system pressure Sometimes
Pressure above 2 bar can also trigger F9. This sometimes happens after a filling loop is left open too long, or if the expansion vessel has lost its pre-charge and can no longer absorb pressure fluctuations during the heating cycle.
- Loose or damaged sensor wiring Sometimes
Wiring between the pressure sensor and the PCB can work loose over time or suffer damage from heat and vibration. A poor connection can mimic a sensor fault, causing the PCB to display F9 even though the sensor itself is intact.
- Pump failure or airlock Sometimes
A seized circulation pump, or a significant airlock within the system, prevents water from moving around the circuit properly. This can cause pressure readings to behave erratically and may contribute to an F9 fault alongside other symptoms such as cold radiators.
- Faulty expansion vessel Rare
The expansion vessel absorbs the natural pressure increase as water heats up. If its internal diaphragm has failed or the pre-charge pressure is too low, system pressure can spike and then fall unpredictably during a heating cycle, repeatedly triggering the pressure sensor.
- System sludge or internal blockage Rare
Heavy sludge build-up in the heat exchanger, pipework, or pump can restrict flow and cause localised pressure anomalies. This is more common in older systems that have never had a power flush or a magnetic filter fitted.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Locate the pressure gauge — on most Glow-Worm models it is on the front fascia or underneath the boiler. A healthy reading is between 1 and 1.5 bar (typically in the green zone). If the needle is below 1 bar, low pressure is the likely cause. If it is above 2 bar, the pressure is too high — see the step on bleeding radiators below.
- Top up system pressure using the filling loop DIY safe
Make sure the boiler is switched off and the system is cold before starting. Locate the filling loop — a braided flexible hose with one or two valves, usually found beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop when the needle reaches 1.2–1.5 bar, then close the valve(s) firmly. Never leave the filling loop open unattended, and do not exceed 1.5 bar.
- If pressure is too high, bleed a radiator to reduce it DIY safe
Turn the boiler off and let it cool. Use a radiator bleed key on the bleed valve (usually at the top corner of a radiator) and open it a quarter turn. Allow a small amount of water to escape until pressure on the gauge drops to around 1–1.5 bar, then close the valve. Have a cloth ready as a small amount of water will come out once the air has been released.
- Reset the boiler DIY safe
Once pressure is within the correct range, press and hold the reset button (usually marked with a flame and a line, or labelled 'Reset') for around 3 seconds, or as described in your boiler's user guide. Allow the boiler a couple of minutes to restart and run through its ignition sequence. If the F9 code clears and the boiler fires up normally, monitor pressure over the next few days.
- Check whether pressure drops again within a short period DIY safe
A one-off pressure drop is not unusual. However, if pressure falls back below 1 bar within a few days of repressurising, or if the boiler needs repressurising more than once every few weeks, this strongly suggests a leak or a faulty component somewhere in the system. Do not continue to top up indefinitely — investigate the root cause.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If F9 returns after repressurising and resetting, or if pressure was already normal when the fault appeared, the cause is likely a faulty pressure sensor, wiring fault, pump issue, expansion vessel failure, or a hidden leak. A Gas Safe registered engineer will inspect the sensor and its wiring, test the expansion vessel, check for leaks with appropriate equipment, and carry out any necessary repairs or replacements. Do not attempt to bypass the pressure sensor or any other safety device.
Parts you may need
- Water pressure sensor · from £25
- Filling loop assembly · from £15
- Expansion vessel (replacement) · from £55
- Expansion vessel Schrader valve / re-charge kit · from £8
- Circulation pump · from £75
- Magnetic system filter (e.g. Fernox TF1 or Magnaclean) · from £55
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F9 fault myself?
If low pressure is the cause, yes — topping up via the filling loop and resetting the boiler is a straightforward homeowner task and costs nothing. However, if the fault returns repeatedly, or if pressure was already correct when F9 appeared, the underlying cause (a faulty sensor, wiring fault, leak, or expansion vessel issue) needs a Gas Safe registered engineer. Never attempt to open or modify any gas or sealed-system components yourself.
How often should I need to top up my boiler pressure?
Most sealed central heating systems lose a small amount of pressure over months, so topping up once or twice a year is within normal range. If you find yourself adding pressure more than once a month, or if the gauge drops noticeably within days of repressurising, there is likely a leak somewhere — in the pipework, at a radiator valve, at the boiler itself, or through the pressure relief valve. An engineer should investigate before permanent damage occurs.
Could the F9 fault be caused by a faulty pressure sensor rather than actual pressure problems?
Yes. If your gauge shows a reading in the normal 1–1.5 bar range but F9 still appears, the pressure sensor is a strong suspect. Sensors can become clogged with sludge from the system water, or they can simply fail electrically. A Gas Safe engineer can test the sensor's output against the actual pressure and replace it if needed — typically costing £150–£300 including labour. It is also worth having a magnetic system filter fitted at the same time to reduce future sludge build-up.
What happens if I ignore the Glow-Worm F9 fault code?
The boiler locks out precisely to prevent damage — continuing to force it to run (for example by repeatedly resetting without fixing the root cause) risks overheating the heat exchanger, stressing the pump, and causing more expensive damage. A cracked heat exchanger, for instance, can cost £400–£600 or more to replace and may make an older boiler uneconomical to repair. Addressing F9 promptly almost always results in a cheaper fix.