A data protocol converter has two interfaces, one of which can be connected to a standard vehicle diagnostic interface, and the other permitting wireless message transfer using a diagnostic handset. A data protocol translator converts the messages from the standard vehicle diagnostic interface into messages having a wireless data format, so that they can be received in the diagnostic handset using the latter's wireless interface. The data protocol converter has a further interface so that it can be connected to a second standard diagnostic interface according to a different standard. The data protocol translator can convert the messages from the second standard vehicle diagnostic interface into messages having a wireless data format.