What to check, in order
- Don't try to solve it yourself
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 defective wiring harness, a triggered flue gas temperature monitor (where fitted), or a break in the plug or wiring of the flue gas safety cut-out.
A repair visit will usually check the pump, the flow of water round the system and the overheat thermostat.
- 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 F25 fault code mean?
The exhaust gases are too hot, or the flue gas safety circuit has been broken, so the boiler has shut down for safety.
Can I fix Vaillant F25 myself?
Engineer only (engineer visit). Start with: don't try to solve it yourself; call a Gas Safe registered engineer. Vaillant lists a defective wiring harness, a triggered flue gas temperature monitor (where fitted), or a break in the plug or wiring of the flue gas safety cut-out.
Is Vaillant F25 worth repairing?
Deal with it now. It's a safety shutdown about the exhaust gases. Have it checked rather than resetting it.
Checked against
- Vaillant: boiler fault codes list
- Vaillant ecoTEC plus open vent installation and servicing instructions (PDF)
- Vaillant ecoFIT pure combi installation and maintenance 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.