System Overview

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

OBD II regulations require that vehicle's on-board computer lights up malfunction indicator light (MIL) on instrument panel when computer detects a malfunction in emission control system/components or in powertrain control components which affect vehicle emissions, or a malfunction in computer. In addition to MIL lighting up when a malfunction is detected, applicable diagnostic trouble code (DTC) prescribed by SAE J2012 is recorded in ECM memory. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If malfunction does not reoccur in 3 consecutive trips, MIL goes off automatically but DTCs remain in ECM memory.

To check DTCs, connect OBD II scan tool or hand-held tester to data link connector No. 3 (DLC3) on vehicle. OBD II scan tool or hand-held tester also enables you to erase DTCs and check freeze frame data and various forms of engine data. For operating instructions, see 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 SAE, while manufacturer controlled codes can be set freely by manufacturer within prescribed limits. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .