What to check, in order
- Check the pressure gauge
Vaillant lists low system pressure and too little water among the causes. With the system cold, its ecoTEC plus manual wants 1.0 to 2.0 bar.
- Top up if it's low
Open the filling loop slowly until the gauge reaches the right pressure, then close it again.
When to call a Gas Safe engineer
If the pressure is fine, or F75 comes back, call a Gas Safe registered engineer. Vaillant lists a faulty pressure sensor, pump or internal expansion vessel, air in the boiler and wiring faults.
A repair visit will usually check the pressure sensor, the pump, the expansion vessel and the pressure relief valve.
Before you book
- Note the model and serial number. They're on a label on the data badge, usually under the boiler or behind the front cover.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Checked against
- Vaillant: boiler fault codes list
- Vaillant ecoTEC plus VU, VUW, VUI ..6/5-5 operating instructions (PDF)
Last reviewed September 2026. Manufacturers update their pages, and your manual is the final word.