DIAGNOSTIC EQUIPMENT

OBD II is a much used and often overused acronym in the repair industry, but just what does it mean, and how will it and its development affect diagnostics? The following guide to OBD II from Vetronix describes the system.

WHAT IS OBD II?

The California Air Resource Board (CARB) required that, by model year 1996, all vehicles sold in California (under 8500 GVWR) contain a certain minimum “On-Board Diagnostic” capability to diagnose emissions-related failures of the engine control system. These diagnostic requirements have been designated as OBD II (On-Board Diagnostics, Phase II) with a goal of monitoring “all of the emissions-related components on-board the vehicle for proper operation.” CARB’s intent with the OBD II program is to “permit the State’s Inspection and Maintenance Program to evolve into a check of the on-board computer for the presence of Diagnostic Trouble Codes (DTCs); under the hood and tailpipe inspections would no longer be required.”

CARB has left the task of defining the Standards and Recommended Practices needed to implement their OBD II requirements to the Society of Automotive Engineers (SAE). The SAE committees that define these documents are made up of worldwide representatives from the vehicle manufacturers, tool manufacturers, service industry, and regulatory agencies, plus other related automotive disciplines.

Part of the OBD II program is for the vehicle to provide a standard interface for off-board diagnostic test equipment. This standard interface includes a standard test connector (referred to as the J1962 connector), a standard communication protocol (SAE J1850 or ISO 9141-2), and a standard set of diagnostic test modes (defined by SAE J1979).

THE OBD II SCAN TOOL

Part of the intent of CARB’s OBD II program is that a single diagnostic tester can be used to read the diagnostic information from any OBD II-compliant vehicle. A tester which satisfies this requirement can be designated as an OBD II Scan Tool and is defined by the SAE document J1978. One of the requirements of the OBD II Scan Tool is that it has to work with any OBD II-compliant vehicle and be able to automatically determine all information required to communicate with the vehicle. The operator does not enter any vehicle-specific information such as the vehicle manufacturer, model year, or engine. The OBD II Scan Tool determines what protocol the vehicle uses, what diagnostic parameters can be read from the vehicle, and what tests are supported by the vehicle.

The SAE Recommended Practice J1978 defines the basic requirements for an OBD II Scan Tool. SAE Recommended Practice J1979 defines the test modes used by both the vehicle and the OBD II Scan Tool to access vehicle information and functions. The following test modes are required for a basic OBD II Scan Tool:

Display Current Diagnostic Data Parameters (J1979, Mode 1).

Display Diagnostic Trouble Codes, DTCs (J1979, Mode 3).

Display Freeze Frame Data captured by the vehicle along with a DTC (J1979, Mode 2).

Clear DTCs, Freeze Frame Data, and any other diagnostic information from the on-board controller’s memory (J1979, Mode 4).

Display results of Oxygen Sensor Monitoring Tests (J1979, Mode 5).

Display results of on-board monitoring tests, both Continuous and Non-Continuous (J1979, Modes 6 and 7).

Display status of the vehicle’s on-board Readiness Tests.