Disclosed are methods and control units for facilitating diagnosis for a vehicle. The invention comprises: receiving configuration information related to a configuration of an add-on interface from a vehicle external communication unit, the add-on interface being arranged in the vehicle as an interface between a vehicle internal system and the add-on system; configuring the add-on interface based on the configuration information; creating, based on the configuration information, at least one diagnostic identifier related to the add-on system; and providing diagnostic information corresponding to the at least one diagnostic identifier to the vehicle external communication unit, thereby facilitating diagnosis of the add-on system performed by at least one diagnosis tool associated with the external communication unit.