Description of On-Board Diagnostics

DIAGNOSIS SYSTEM




- When troubleshooting the OBD II vehicles, the only difference from the usual troubleshooting procedure is that you connect the vehicle to the OBD II scan tool complying with the SAE J1987 or the hand-held tester, and read off the various data output from the vehicle's ECM.

- The OBD II regulations require that the vehicle's on-board computer should light up the Malfunction Indicator Lamp (MIL) on the instrument panel when the computer detects a malfunction in the computer itself or in the drive system components which affect the vehicle emissions. In addition to the MIL lighting up when a malfunction is detected, the applicable DTCs prescribed by the SAE J2012 are recorded in the ECM memory.

If the malfunction only occurs in 3-trips, the MIL goes off but the DTCs remain recorded in the ECM memory.




- To check the DTCs, connect the scan tool (OBD II scan tool or hand-held tester) to the DLC3 on the vehicle. The scan tool also enables you to erase the DTCs and check the freeze frame data and various forms of the engine data (For instruction book).

- The DTCs include the SAE controlled codes and the Manufacturer controlled codes.

The SAE controlled codes must be set as prescribed by the SAE, while the Manufacturer controlled codes can be set freely by a manufacturer within the prescribed limits (See the DTC chart).

- The diagnosis system operates in the normal mode during the normal vehicle use, and also has the check mode for technicians to simulate malfunction symptoms and perform the troubleshooting. Most DTCs use the 2-trip detection logic* to prevent erroneous detection. By switching the ECM to the check mode when troubleshooting, the technician can cause the MIL to light up for a malfunction that is only detected once or momentarily (hand-held tester).

- *2-trip detection logic
When a logic malfunction is first detected, the malfunction is temporarily stored in the ECM memory. If the same malfunction is detected again during the 2nd test drive, this 2nd detection causes the MIL to light up.