What to check, in order
- Don't try to repair it yourself
Vaillant's user manual says fault codes like this are for a competent person, and never to carry out repairs on the boiler yourself.
- Call a Gas Safe registered engineer
Or Vaillant on 0344 736 0049. Have the boiler's model and the code ready.
When to call a Gas Safe engineer
Vaillant's installation manuals list a sensor plug that has come loose or isn't plugged in, a break in the wiring, or a faulty sensor.
A repair visit will usually check the sensor's reading and its wiring.
- 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 F00 fault code mean?
The boiler can't get a proper reading from the sensor that measures the water leaving it, so it has stopped for safety.
Can I fix Vaillant F00 myself?
Engineer only (engineer visit). Start with: don't try to repair it yourself; call a Gas Safe registered engineer. Vaillant's installation manuals list a sensor plug that has come loose or isn't plugged in, a break in the wiring, or a faulty sensor.
Is Vaillant F00 worth repairing?
Worth fixing. A temperature sensor or its plug is a routine repair for an engineer.
Checked against
- Vaillant ecoTEC plus open vent installation and servicing instructions (PDF)
- Vaillant ecoFIT pure combi installation and maintenance instructions (PDF)
- Vaillant ecoTEC plus ..6/5-5 user manual (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.