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.
- 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.
Quick answers
What does the Vaillant F23 fault code mean?
The water leaving the boiler is much hotter than the water coming back to it. Vaillant's causes point to water not circulating as it should.
Can I fix Vaillant F23 myself?
Engineer only (5 minutes of checks). Start with: check the pressure gauge; call an engineer if it returns. 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.
Is Vaillant F23 worth repairing?
Get a quote first. The pump and the diverter valve are both on Vaillant's list, so get a price before the work starts.
Checked against
- Vaillant: boiler fault codes list
- Vaillant ecoTEC plus VU, VUW, VUI ..6/5-5 operating instructions (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.