What to check, in order
- Don't keep resetting it
Vaillant says this is a case for an expert, and trying to solve it yourself can be dangerous.
- 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 lists a gas solenoid valve that isn't sealing properly, or a faulty circuit board (PCB), as the causes.
A repair visit will usually check the gas supply, the ignition electrode and the flame sensor.
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
Last reviewed September 2026. Manufacturers update their pages, and your manual is the final word.