ISO 27145-6:2015 defines the requirements for the external test equipment as listed:

- a means of establishing communications between a WWH-OBD-equipped vehicle and external test equipment;

- a set of diagnostic services, including addressing methods, to be provided by the external test equipment in order to exercise the services defined in ISO 27145‑3.

ISO 27145-6:2015 describes the minimum capabilities or functions in the external test equipment. Additional functionality, e.g. non WWH-OBD protocols or retrieval of repair and maintenance information, can be integrated into the external test equipment according to the test equipment manufacturer needs. The external test equipment designer ensures that no such capability or function can adversely affect either a WWH-OBD-equipped vehicle connected to the equipment, or the equipment itself.

When the external test equipment implements functionality, which is not covered by ISO 27145‑3, this functionality is not linked to the timing requirements defined in this International Standard.