Smell gas? Leave the property and call the National Gas Emergency line on 0800 111 999

Viessmann fault code

Viessmann F3: flame signal already present when the burner starts

Engineer only

The boiler detected a flame signal before it lit, which points to the flame-sensing electrode or its cable.

What you can safely do

There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.

Find a Gas Safe engineer near you Opens the official Gas Safe Register search.

Applies to: Vitodens 100-W (B1HC/B1KC). Codes can differ between models, so check your manual.

From Viessmann's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

Model	Meaning	Causes listed for engineers
Vitodens 100-W (B1HC/B1KC)	Flame signal is already present at burner start	Check ionisation electrode and connecting cable

Is F3 dangerous?

Leave it off and book an engineer. If you smell gas, call 0800 111 999.