System Overview

When trouble shooting OBD II vehicles, the only difference from the usual trouble shooting procedure is that you connect to the vehicle the OBD II scan tool complying with SAE J1978 or the hand-held tester, and read off various data output from the vehicle's ECM.

OBD II regulations require that the vehicle's on-board computer lights up the malfunction indicator light (MIL) on the instrument panel when the computer detects a malfunction in the emission control system/components or in the power train control components which affect vehicle emissions, or a malfunction in the computer. In addition to the MIL lighting up when a malfunction is detected, the applicable diagnostic trouble code (DTC) prescribed by SAE J2012 are recorded in the ECM memory. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

If the malfunction does not reoccur 3 consecutive times, the MIL goes off automatically but the DTCs remain recorded in the ECM memory.

To check the DTCs, connect the OBD II scan tool or the hand-held tester to DLC3 on the vehicle. The OBD II scan tool or the hand-held tester also enables you to erase the DTCs and check the freeze frame data and various forms of engine data. For operating instructions, see the OBD II scan tool's instruction book. DTCs include SAE controlled codes and manufacturer controlled codes. SAE controlled codes must be set as prescribed by the SAE, while manufacturer controlled codes can be set freely by the manufacturer within the prescribed limits. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .