What to check, in order
- Check the pressure gauge
Too little water in the boiler is one of the causes Vaillant lists. With the system cold, its ecoTEC plus manual wants 1.0 to 2.0 bar.
- Top up only if it's low
Use the filling loop to bring it back into range. Leave everything else to an engineer.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer. Vaillant's list of causes includes a faulty pump, flow or return temperature sensor, diverter valve, pressure sensor or flow sensor.
A repair visit will usually check the pump, the flow of water round the system and the overheat thermostat.
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.