I used to own a Vento and enjoyed being able to read the error codes using an OBD2 interface.
Now I wanted to ask you if it's possible to do the same thing on my Peugeot 406 (manufactured in 2001), since the connector (at least visually) seems to be the same as the one on the Vento.
Yes, OBD2 should work on all 406 models from the year 2000 onwards, regardless of whether they are gasoline or diesel engines.
An interface with ISO9141-2 capability is sufficient, such as one of the semi-professional diagnostic systems with KKL support.