F.75 on a Glow-worm is a pressure sensor fault. The boiler is not getting a usable reading from the water pressure sensor.

Worth checking your pressure gauge first, because a genuinely low system pressure and a faulty sensor can look similar from the outside.

What F.75 actually means

The boiler uses the pressure sensor to confirm there is water in the system and to detect the pressure change the pump produces when it starts.

If the sensor does not report the expected change, the boiler cannot confirm that water is present and circulating, and it stops.

What you can safely check yourself

Look at the pressure gauge. If it is well below normal, roughly under 1 bar, the system is genuinely low on water.

Before topping it up, ask why it dropped. A sealed system should not lose pressure, and repeatedly refilling it introduces fresh oxygenated water, which causes sludge over time.

If the pressure is correct on the gauge and the boiler still reports F.75, that points at the sensor or the pump rather than the water.

When to stop, and why

What causes it beyond the simple checks

A failed pressure sensor, a blocked sensor port, a damaged or corroded connector, wiring damage, or a pump that is not producing the expected pressure change because it is failing or air locked. Glow-worm is part of the Vaillant Group and the Energy range shares its fault code structure with Vaillant appliances.

How Vantage can help

We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.

Frequently asked questions

Is F.75 the same as low pressure?

Not quite. Low pressure is the system genuinely lacking water. F.75 is the boiler being unable to trust the sensor, which can be caused by low pressure, a failed sensor or a pump problem.

Can I fix it by topping up?

If the pressure was genuinely low, that may clear it. If the gauge reads normal, topping up will not help and may overpressurise the system.

Could it be the pump?

Yes. The sensor is looking for a pressure change when the pump runs, so a failing pump can present as a sensor fault.

Related fault codes

Other Glow-worm codes: F.20 · F.22 · F.23 · F.24 · F.25 · F.27 · F.28 · F.29 · F.32 · F.61 · all boiler advice. See also boiler repair. The same family on other makes: Vaillant F75 · Ideal F1 · Worcester Bosch A1. See also boiler keeps losing pressure.

In Glow-worm’s own words

Glow-worm use different code sets across their ranges and generations. The definition above comes from The Glow-worm Energy range fault finding table. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.

When to call an engineer

If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.

Need help today?

Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.