Diagnosis System

DESCRIPTION OF ON-BOARD DIAGNOSIS




When troubleshooting OBD II vehicles, the only difference from the usual troubleshooting procedure is that you connect to the vehicle the OBD II scan tool complying with SAE J1978 or LEXUS 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 Lamp (MIL) on the instrument panel when the computer detects a malfunction in the computer itself or in drive system components which affect vehicle emissions. In addition to the MIL lighting up when a malfunction is detected, the applicable Diagnostic Trouble Codes (DTC) prescribed by SAE J2012 are recorded in the ECM memory .

If the malfunction has been replaced, the MIL goes off automatically but the DTC remain recorded in the ECM memory.




To check the DTC, connect the OBD II scan tool or LEXUS hand-held tester to Data Link Connector 3 (DLC3) on the vehicle. The OBD II scan tool or LEXUS hand-held tester also enables you to erase the DTC and check freeze frame data and various forms of engine data (For operating instructions, see the OBD II scan tool's instruction book.).

DTC 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 DTC chart).

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

If troubleshooting priorities for multiple DTC are given in the applicable DTC chart, these should be followed.

If no instructions are given troubleshoot DTC according to the following priorities.
1. DTC other than fuel trim malfunction (DTC P0171, P0172), EGR (DTC P0401, P0402) and misfire (DTC P0300 - P0306).
2. Fuel trim malfunction (DTC P0171, P0172) and EGR (DTC P0401, P0402).
3. Misfire (DTC P0300 - P0306).