What to check, in order
- Check the pressure gauge
With the system cold, Vaillant's ecoTEC plus manual wants 1.0 to 2.0 bar. Top up with the filling loop if it's low.
- Call an engineer if it returns
Vaillant says F23 is a case for an expert.
When to call a Gas Safe engineer
Vaillant's list of causes includes air in the boiler, a weak or blocked pump, a faulty or blocked diverter valve, faulty flow or return sensors and a faulty expansion vessel.
A repair visit will usually check the pump and the difference between the flow and return temperatures.
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.