ISO 27145-3:2017 — WWH-OBD — Part 3: Common Message Dictionary

1. Scope and Overview of WWH-OBD Communication

ISO 27145-3:2012 defines the common message dictionary for World-Wide Harmonized On-Board Diagnostics (WWH-OBD) communication requirements. This part of the ISO 27145 series specifies unified diagnostic services (UDS) applicable to WWH-OBD including message formats, service identifiers, data parameters, application layer protocol, presentation layer formatting, and session layer management for vehicle diagnostic communication. WWH-OBD represents a landmark harmonization of on-board diagnostic regulations across major automotive markets including Europe, North America, Japan, Korea, and China, enabling a single vehicle design to satisfy multiple regulatory requirements worldwide.

The standard specifically defines how electronic control units (ECUs) should respond to diagnostic requests from external test equipment. It covers four essential diagnostic services: ReadDataByIdentifier (service ID 0x22) for reading sensor values, calculated parameters, and system status information; ReadDTCInformation (0x19) for retrieving diagnostic trouble codes with associated status and freeze frame data; ClearDiagnosticInformation (0x14) for resetting DTCs and monitor status after repairs; and RoutineControl (0x31) for initiating or stopping onboard diagnostic tests such as oxygen sensor heater monitoring or EVAP system leak tests. These standardized services ensure consistent diagnostic interfaces across all vehicle manufacturers.

2. Message Structure and Protocol Requirements

The common message dictionary establishes standardized message formats including request and response message structures, timing parameters, and addressing schemes. Physical addressing enables point-to-point communication with a specific ECU for detailed diagnostics, while functional addressing allows broadcast communication to all ECUs for global requests such as clearing DTCs across all systems simultaneously. The standard specifies detailed timing requirements for both server (ECU) response time (P2 parameter) and client (test equipment) response time (P3 parameter), which are critical for ensuring interoperability between vehicles and diagnostic tools from different manufacturers. Typical P2 timing for WWH-OBD is 50 milliseconds for standard responses.

The application layer supports segmented data transfer for large data sets through the ReadDataByIdentifier service — when requested data exceeds the maximum message length, the ECU automatically segments the response and the test equipment reassembles the segments. The presentation layer defines data conversion rules for scaling and translating raw sensor values into engineering units using standardized conversion formulas and lookup tables. The session layer manages diagnostic sessions including default session, programming session, and extended diagnostic session, each with different timing parameters and service availability. Session timeout (S3 parameter) automatically terminates extended sessions if no diagnostic communication occurs within the specified period.

The standard specifies the relationship between ISO 27145-3 and the base UDS protocol defined in ISO 14229-1. WWH-OBD uses a subset of the full UDS service set with specific constraints on message length, timing parameters, and data format to ensure global harmonization. Part 3 also references the common data dictionary defined in ISO 27145-2 which provides standardized definitions for all data parameters that can be accessed through diagnostic services, including sensor readings, calculated values, system status information, and monitor test results.

3. Engineering Insights for Automotive Diagnostic Systems

Implementing ISO 27145-3 in ECU firmware requires careful attention to the relationship between this standard and the UDS protocol in ISO 14229-1. The WWH-OBD subset of UDS services is intentionally limited to ensure universal compatibility, but this means some advanced diagnostic capabilities available in full UDS may not be accessible through WWH-OBD alone. Vehicle manufacturers may implement both WWH-OBD and manufacturer-specific UDS services in the same ECU, with the WWH-OBD services providing the regulated minimum diagnostic functionality and proprietary services providing enhanced capabilities for dealer-level diagnostics.

The informative annexes in the standard provide practical examples of diagnostic service sequences that are invaluable for developers. These examples demonstrate complete message flows for reading DTCs with status information, accessing freeze frame data captured at the time of fault occurrence, performing oxygen sensor monitor tests, and conducting comprehensive system monitoring tests. Each example includes the exact request and response message bytes with annotations explaining the function of each byte. Understanding these examples thoroughly is essential for correct implementation, as they illustrate the intended use of each service in real diagnostic scenarios.

Software developers implementing the protocol in either ECU firmware or diagnostic scan tool software should use the standard’s detailed message format specifications as the primary reference. The negative response codes defined in the standard provide important diagnostic information when a request cannot be processed — common codes include serviceNotSupported, requestOutOfRange, and conditionsNotCorrect. Proper handling of these negative responses is essential for robust diagnostic communication. The standard also specifies sub-function parameters within each service that modify the service behavior for specific use cases, such as requesting DTCs by status mask or requesting freeze frame data for a specific DTC.

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