Vaillant F.75 Fault Code
No pressure change detected when the pump starts — typically a faulty pump or water pressure sensor.
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 Vaillant F.75 fault code mean?
No pressure change detected when the pump starts — typically a faulty pump or water pressure sensor.
Common causes
- Faulty water pressure sensor Common
The most common cause — the sensor fails to register the pressure rise when the pump runs.
- Faulty or seized pump Common
If the pump is not building pressure, the boiler reads no change and locks out.
- Low system pressure or trapped air Sometimes
Low pressure or air in the system can trigger the fault; topping up and bleeding may help temporarily.
How to fix it
- Check and top up system pressure to ~1.2 bar. DIY safe
Use the filling loop to bring pressure into the normal range, then reset the boiler.
- Call a Gas Safe registered engineer. Gas Safe engineer
F.75 usually points to a failed pressure sensor or pump, which require diagnosis and replacement by a qualified engineer.
Parts you may need
- Water pressure sensor · from £40
- Circulation pump · from £120
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 £120–£400, depending on the underlying cause.
Frequently asked questions
What is the most common cause of a Vaillant F.75 fault?
A faulty water pressure sensor is the most common cause, often alongside a failing pump.