Vaillant F.75 Fault Code: Causes, Fixes & Repair Costs
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
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.