Description of On-Board Diagnostics

DIAGNOSIS SYSTEM

When troubleshooting On-Board Diagnostic (OBD II) vehicles, the vehicle must be connected to the OBD II scan tool (in compliance with SAE J1987) or the hand-held tester. Various data output from the vehicle's ECM can then be read.




OBD II regulations require that the vehicle's on-board computer illuminates the Malfunction Indicator Lamp (MIL) on the instrument panel when the computer detects a malfunction in: 1) the computer, or 2) drive system components (which affect the vehicle emissions). In addition, the applicable DTCs prescribed by SAE J2012 are recorded in the ECM memory.

If a malfunction only occurs in 3 consecutive trips, the MIL turns off but the DTCs remain in the ECM memory.




- To check the DTCs, connect the OBD II scan tool or hand-held tester to the Data Link Connector 3 (DLC3) of the vehicle. The OBD II scan tool or hand-held tester also enables you to erase the DTCs and check freeze frame data and various forms of engine data (see the tester's instruction book for further details).
- The DTCs include SAE controlled codes and manufacturer controlled codes. SAE controlled codes must be set according to the SAE, while manufacturer controlled codes can be set freely by a manufacturer with certain restrictions (see the DTC chart ).
- The diagnosis system operates in "normal mode" during normal vehicle use. In "normal mode", 2 trip detection logic is used to ensure accurate detection of malfunctions. "Check mode" is also available to technicians as an option. In "check mode", 1 trip detection logic is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent malfunctions (hand-held tester only).
- When a malfunctions is first detected, the malfunctions is temporarily stored in the ECM memory (1st trip). If the ignition switch is turned OFF and then ON again, and the same malfunction is detected again, the MIL will illuminate (2nd trip).