Glow-Worm F75 Fault Code: Pressure Sensor Fault
The F75 code appears when the boiler's pressure sensor fails to detect any change in water pressure at the moment the circulation pump starts up. In normal operation, when the pump kicks in it creates a small but measurable shift in system pressure — the sensor picks this up and signals the PCB that water is moving safely. When that expected shift doesn't register, the boiler treats it as a circulation failure and locks out to prevent the heat exchanger from overheating with static water. You'll lose both heating and hot water until the fault is cleared.
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 F75 fault code mean?
The F75 code appears when the boiler's pressure sensor fails to detect any change in water pressure at the moment the circulation pump starts up. In normal operation, when the pump kicks in it creates a small but measurable shift in system pressure — the sensor picks this up and signals the PCB that water is moving safely. When that expected shift doesn't register, the boiler treats it as a circulation failure and locks out to prevent the heat exchanger from overheating with static water. You'll lose both heating and hot water until the fault is cleared.
Common causes
- Faulty or degraded pressure sensor Common
Over years of use — typically beyond the 7–10 year mark — the pressure sensor's internal components wear out or corrode, particularly in areas with hard or untreated water. It stops sending accurate readings to the PCB and triggers F75 even when system pressure is perfectly normal.
- Sensor blocked with sludge or limescale Common
The sensor reads pressure through a small internal port. In older systems or those without a magnetic filter, this port can become clogged with magnetite sludge, rust particles, or limescale (especially in hard-water areas like the South East). A blocked port mimics a failed sensor — the boiler sees no pressure change and locks out.
- Low system water pressure Sometimes
If the system pressure has dropped below around 0.5–0.8 bar — due to a slow leak, air in the system, or radiator bleeding without topping back up — there may not be enough baseline pressure for the sensor to register the pump's effect. The boiler cannot confirm safe circulation and raises F75.
- Seized or failing circulation pump Sometimes
If the pump itself has stopped working properly — seized bearings, a failed capacitor, or internal wear — water simply isn't moving when the pump should activate. With no circulation there's no pressure shift, and the sensor correctly reports that nothing is happening. The fault code appears but the root cause is mechanical pump failure rather than the sensor itself.
- Faulty expansion vessel Rare
A waterlogged or under-pressurised expansion vessel can cause erratic pressure behaviour throughout the system. If the vessel's diaphragm has failed, pressure may spike and drop unpredictably, giving the sensor inconsistent data and occasionally causing spurious F75 lockouts.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button for around five seconds until the display acknowledges the restart. Once the boiler cycles back up, check whether F75 has cleared. If it fires up and runs normally, monitor it over the next few days. If the fault returns, move on to the next step rather than resetting repeatedly — more than two or three resets without a fix is a sign something needs physical attention.
- Check and top up system pressure DIY safe
Look at the pressure gauge on the boiler (or the digital display if your model shows bar readings). A cold system should read between 1 and 1.5 bar. If it's below 1 bar, use the filling loop — typically a braided flexible hose or a built-in lever valve beneath the boiler — to slowly add water until the gauge reads around 1.2–1.5 bar, then close the loop firmly. Reset the boiler afterwards and observe whether F75 clears. If pressure keeps dropping over days, there is likely a leak somewhere in the system that a Gas Safe engineer will need to locate.
- Inspect and thaw the condensate pipe if relevant DIY safe
During cold spells, a frozen condensate pipe can cause unusual fault codes. Check the white plastic pipe that exits the boiler and runs to an external drain — if it feels solid or you can see ice, carefully pour warm (not boiling) water along its length to thaw it, then reset the boiler. This is unlikely to be the sole cause of F75 but is worth ruling out in winter.
- Have a Gas Safe engineer test the pressure sensor Gas Safe engineer
If the homeowner steps haven't resolved the fault, a registered engineer will use a multimeter or manufacturer diagnostic tool to check whether the pressure sensor is sending correct voltage signals to the PCB. If the sensor is reading incorrectly or not at all, the engineer will replace it with a compatible Glow-Worm part and test the boiler under load to confirm stable readings.
- Have a Gas Safe engineer inspect and clean or replace the pump Gas Safe engineer
The engineer will check whether the circulation pump is actually running and generating flow. A seized pump may sometimes be freed by carefully accessing the pump head, but a failed pump will need a full replacement. The engineer will also check that the pump speed setting is appropriate for the system.
- Have a Gas Safe engineer check the expansion vessel Gas Safe engineer
The engineer will measure the vessel's charge pressure with a gauge. A waterlogged vessel needs its diaphragm replaced or the entire vessel swapped out. This is straightforward work for a qualified engineer but requires the system to be drained down safely first.
- Consider a power flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy magnetite or sludge — discoloured system water, cold radiator spots, a clogged sensor port — a power flush using specialist equipment and chemical cleaners can clear the system. A magnetic system filter fitted afterwards will help prevent recurrence. This is engineer-only work involving pressurised flushing equipment.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If F75 keeps returning despite the above work, the PCB itself may be misinterpreting sensor signals. PCB diagnosis and replacement is specialist work that must be carried out by a Gas Safe registered engineer. For boilers over 10–12 years old with recurring faults, it's worth getting a quote for a replacement boiler alongside the repair, as ongoing costs can exceed the value of a very aged appliance.
Parts you may need
- Glow-Worm pressure sensor · from £45
- Circulation pump (compatible replacement) · from £90
- Expansion vessel (8–12 litre) · from £40
- Magnetic system filter (e.g. Adey MagnaClean) · from £55
- Filling loop assembly · from £15
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F75 fault myself?
The two homeowner-safe steps are topping up system pressure via the filling loop and resetting the boiler. If either of those clears F75 and it stays away, great. But replacing the pressure sensor, working on the pump, or servicing the expansion vessel all require a Gas Safe registered engineer — both legally and for your safety. Don't attempt to open the boiler casing or touch internal components.
How much does it cost to fix a Glow-Worm F75 fault in the UK?
Most people pay somewhere between £150 and £380 depending on the cause. A straightforward sensor replacement typically comes in around £150–£220 including parts and labour. A pump replacement is usually £250–£350 fitted. If the system needs a full power flush on top of component work, costs can rise towards £600–£800 in total — though that's an outlier rather than the norm. PCB replacement, if it ever comes to that, can cost £300–£500 for parts alone, which is worth weighing against the age of the boiler.
Why does my Glow-Worm keep showing F75 after I reset it?
A fault that clears on reset but keeps returning means the underlying cause hasn't been addressed. The most common culprits are a pressure sensor that's failing intermittently, a pump that struggles to get up to speed before the boiler times out, or gradually dropping system pressure from a slow leak. Each reset buys a little time but doesn't fix anything. More than two or three resets without a lasting result is your cue to book an engineer before the boiler locks out completely.
Is the Glow-Worm F75 fault dangerous?
The lockout itself isn't dangerous — it's the boiler protecting itself from overheating with static water. You'll simply have no heating or hot water until it's resolved. However, you shouldn't keep forcing the boiler to run by repeatedly resetting it if the pump or sensor is genuinely faulty, as that can stress the heat exchanger over time. Get it looked at promptly, particularly in cold weather when you need the heating most.