Glow-worm F75 Fault

Glow-worm F75 Fault Code Repair: Pressure Sensor Problem

Last updated on January 9th, 2024

An F75 Glow-worm fault code show on the boilers screen when there is a problem and your boiler locks out meaning you’ll have no heating or hot water.

Glow-worm uses them to show you what might be wrong with the boiler when it’s not working.

Having a fault code on the screen is a great way to help diagnose your boiler problems.

I have been carrying out boiler repairs for over a decade and having fault codes on the screen when I arrive is very welcome.

Not all boilers use fault codes, so having them is a huge bonus.

What Does F75 Mean on a Glow-worm Boiler?

The Glow-worm boiler F75 fault code (like the Vaillant F75) most of the time means you have a blocked or faulty pressure sensor.

Glow-worm F75 pressure sensor
Glow-worm pressure sensor

The pressure sensor measures the pressure of your central heating system.

Water pressure sensors can get blocked with dirt and debris from a dirty central heating system.

A blocked pressure sensor means the pressure gauge will not move when you repressurise your Glow-worm boiler.

This will cause your boiler to lock out and show the F75 fault code to prevent damaging the boiler.

How to Fix F75 Fault on a Glow-worm Boiler

To fix the F75 fault on a Glow-worm boiler, you must replace the faulty pressure sensor.

Sometimes you can remove and clean the old dirty sensor, but most of the time it needs to be replaced.

Make Sure it’s Clean

You should clean the full system when replacing the sensor as a new water pressure sensor will probably get blocked again on a dirty central heating system.

Having your system flushed and fitted with a magnetic system filter will clean and protect your central heating system for many years to come. This will prevent sludge problems like a blocked pressure sensor.

Glow-worm F75 Pressure Sensor Replacement

How to replace the water pressure sensor on a Glow-worm F75 fault:

  1. Isolate the electric supply to the boiler
  2. Isolate the flow and return valves under the boiler and drain the boiler from a drain off on the flow or return. Or, you can drain the central heating system
  3. Remove the casing from the boiler and pull down the panel
  4. Remove the screw on the black plastic air inlet pipe then pull away the pipe
  5. Remove the clip from the sensor and pull the sensor away from the pipe
  6. Remove the electrical connection
  7. Connect the electrical connector to the new pressure sensor
  8. Fit the new sensor and refit the clip
  9. Refit the casing on the boiler
  10. Close any drain-off valves
  11. Open the flow and return valves if they were closed
  12. Turn the electric supply back on
  13. Check the pressure and repressurise the boiler if the pressure gauge is showing less than 1 bar
  14. Turn the heating on to make sure all is good

Glow-worm Water Pressure Sensor Location

The Glow-worm water pressure sensor is located on the main copper heating pipe on the left-hand side of the boiler. You have to remove the boiler casing to access it, and it is quite hard to find and access at first.

F75 Glow-worm Fault Code Reset

The first thing I would check on a boiler that is not working, no matter what fault code is showing, is to check the pressure and reset the boiler.

Knowing how to reset a Glow-worm boiler is the simplest way to get your boiler working again as it might have just locked out.

To reset a Glow-worm boiler, you must find the reset button. This is usually a flame with a line through it. You have to press the button to reset the boiler.

Sometimes you have to hold it down for 3 seconds, but on some Glow-worm boilers, it just needs to be pressed.

Glow-worm Boiler Guides


Replacing a pressure sensor on a Glow-worm combi boiler is easy for anyone who has done any kind of boiler repairs before.

The Glow-worm F75 error code is a good sign that your heating system is dirty and should be cleaned after replacing the pressure sensor.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.


How do you clean a water pressure sensor?

To clean a water pressure sensor on a Glow-worm boiler, you will need to drain the boiler and remove the sensor as if you were replacing it.

Once it is removed, you should use a tiny screwdriver or a thin, pointed tool. The dirt that accumulates in a pressure sensors hole, and eventually blocks it, is solid dirt and hard (sometimes impossible) to remove.

That’s why I don’t recommend cleaning a water pressure sensor. You can buy a new one for about £25.

The amount of work involved in cleaning it is more than replacing it, and you might not actually get it to work again. It’s a bit of a gamble putting everything back together for it not to work and having to replace it anyway.

Where is the pressure sensor on a Glow-worm boiler?

The pressure sensor on a Glow-worm boiler is on the main copper heating pipe on the left-hand side of the boiler. You have to remove the boiler casing to access it.

How does a water pressure sensor work?

A water pressure sensor on a boiler works by measuring the force on a diaphragm which will switch the boiler off when the pressure is too low or there’s another problem.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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