This part of ISO 27145 defines all regulatory data elements of the ISO 27145 series. The data elements are used to provide the external test equipment with the diagnostic status of the vehicle on-board diagnostics (VOBD) system in the vehicle. All data elements are communicated with the unified diagnostic services as defined in ISO 27145‑3. The data elements are diagnostic trouble codes (DTCs), data identifiers (DIDs) and routine identifiers (RIDs). The mapping from parameter identifiers (PIDs), monitor identifiers (MIDs) and info type identifiers (ITIDs) is described in this part of ISO 27145.

If new legislated WWH-OBDGTR modules are established, it is intended that ISO 27145 be applicable with possible extensions, which can be included in this part of ISO 27145, can be specified as enhancements of the SAE Digital Annexes or can even be part of other referenced documents which are intended to include the applicable data definitions.

ISO 27145 is intended to become the single communication standard for access to OBD-related information (VOBD). To allow for a smooth migration from the existing communication standards to this future world-wide communication standard, the communication concept as specified in ISO 27145‑4 is based on two different data links: ISO 15765‑4 and ISO 13400 (all parts).

In view of the usage of standard network layer protocols, future extensions to optional physical layers (e.g. wireless) are possible.

ISO 27145‑4 is the entry point to establish communication with the vehicle. Based on the results of the initialization, the external test equipment determines which protocol and diagnostic services are supported by the vehicle's emissions-related system, i.e.

Vehicles according to ISO 27145 provide VOBD system support as envisioned for WWH-OBD by Global Technical Regulation (GTR) No. 5.