Glow-worm F.75 Fault Code
The F.75 fault is a pressure sensor failure — the sensor itself is not reading correctly, rather than the system actually being low on water. This distinguishes it from F.22 (genuine low pressure) and requires engineer diagnosis to confirm which is the cause.
What is the Glow-worm F.75 fault code?
The Glow-worm F.75 fault is triggered when the PCB cannot get a valid pressure reading from the pressure sensor after the pump starts. Unlike F.22, which indicates the system genuinely has insufficient water pressure, F.75 indicates the sensor itself has malfunctioned and is returning an incorrect or absent reading. The PCB cannot determine system pressure and locks out as a safety measure. In some cases the pressure gauge will show a normal reading (1.0–1.5 bar) while F.75 is displayed — this is a clear sign the sensor has failed rather than pressure being genuinely low. A Gas Safe engineer is required to diagnose and replace the sensor.
What causes the F.75 fault?
The sensor's internal transducer fails and stops returning a valid signal to the PCB. The most common cause of F.75 — the sensor has reached end of life. Particularly common in South London where hard water accelerates internal component degradation.
The PCB receives a value from the sensor that falls outside the expected range, triggering F.75 even if the sensor is partially functional. This can manifest as an intermittent fault before becoming permanent.
A loose, corroded, or broken connector between the pressure sensor and PCB can cause the PCB to see no signal or an erratic signal, triggering F.75.
Thames Water calcium deposits can block the sensor's pressure measurement port, preventing accurate readings. More common on Glow-worm boilers that have not been serviced annually.
In some cases, genuinely low pressure and a borderline sensor combine to trigger F.75. An engineer will check actual system pressure at the diagnostic visit to distinguish this from pure sensor failure.
What you can safely check yourself
Check the physical pressure gauge on the boiler front. If it reads 1.0–1.5 bar and the boiler still shows F.75, the sensor itself has failed — not the pressure. Do not repressurise if pressure appears normal.
If the gauge reads below 0.8 bar, repressurise to 1.2 bar and attempt one reset. If the gauge reads normal but F.75 persists, call an engineer.
One reset is acceptable. If F.75 returns immediately, a sensor replacement is required — call a Gas Safe engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Test pressure sensor output signal with a multimeter
- Check sensor wiring harness for corrosion or loose connection
- Confirm actual system pressure with a separate gauge
- Replace pressure sensor if output is out of specification
- Check PCB pressure sensing circuit if sensor tests normal
- Test boiler through a full heating cycle after replacement
Pressure sensor failures are disproportionately common in South London's hard water areas. Thames Water's calcium-heavy supply deposits scale on the sensor port over time, degrading sensor accuracy and eventually causing F.75. Across Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley, Glow-worm F.75 faults are among Reion's most frequent callouts on boilers over 8 years old.
How much does a F.75 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Glow-worm boiler showing F.75
Check the physical pressure gauge on the boiler front panel.
If pressure is below 1 bar, repressurise to 1.2 bar before resetting.
Hold the reset button for 3–5 seconds.
If F.75 returns immediately while the gauge shows 1.0–1.5 bar, the sensor has failed.
Call a Gas Safe engineer — sensor replacement is required.
Pressure sensor replacement is a moderate-cost repair — typically £120–£220 all in. On any Glow-worm boiler under 12 years old it is unambiguously worth doing. Glow-worm uses Vaillant Group sensors, so parts are readily available and delivery is fast.
Common questions about the Glow-worm F.75 fault
What is the difference between Glow-worm F.22 and F.75?
Can I fix a Glow-worm F.75 myself?
How much does a Glow-worm F.75 repair cost in London?
Is a Glow-worm F.75 fault the same as a Vaillant F.75?
Why does my Glow-worm boiler show F.75 only sometimes?
Other Glow-worm fault codes
Need a Glow-worm engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
