Diagnosis System: Description
When troubleshooting On-Board Diagnostic (OBD II) vehicles, the vehicle must be connected to the OBD II scantool (in compliance with SAE J1978). 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 Light (MIL) on the instrument panel when the computer detects a malfunction in:
- The emission control system/components
- The powertrain control components (which affect vehicle emissions)
- The computer.
In addition, the applicable Diagnostic Trouble Code (DTC) prescribed by SAE J2012 are recorded in the ECM memory.
If the malfunction does not reoccur in 3 consecutive trips, the MIL goes off automatically but the DTCs remain recorded in the ECM memory.
To check DTCs, connect the OBD II scantool to the Data Link Connector 3 (DLC3) of the vehicle. The scantool also enables you to erase the DTC and check the Freeze Frame Data and various forms of the engine data (Refer to DIAGNOSIS TROUBLE CODE CHART on ).