Description of On-Board Diagnostics

DESCRIPTION:
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. As in previous models, when the ECM detects a malfunction in the transaxle control section. the MIL lights up and the O/D OFF indicator light blinks. In addition to the MIL lighting up when a malfunction is detected, the applicable diagnostic trouble codes prescribed by SAE J2012 are recorded in the ECM memory.
If the malfunction only occurs in 3 trips, the MIL goes off but the diagnostic trouble codes remain recorded in the ECM memory. The O/D OFF indicator light goes off as soon as the trouble is no longer detected.




To check the diagnostic trouble codes, connect the OBD II scan tool or LEXUS hand-held tester to Data Link Connector 3 on the vehicle. The OBD II scan tool or LEXUS hand-held tester also enables you to erase the diagnostic trouble codes and check freeze frame data and various forms of engine data.

NOTE: (For operating instructions, see the OBD II scan tool's instruction book.)

Diagnostic trouble codes 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.

The diagnosis system operates in normal mode during normal vehicle use, and also has a check mode for technicians to simulate malfunction symptoms and perform troubleshooting. Most diagnostic trouble codes use 2 trip detection logic (*) to prevent erroneous detection. By switching the ECM to check mode when troubleshooting, the technician can cause the MIL to light up and O/D OFF indicator lights to blink for a malfunction that is only detected once or momentarily. (LEXUS 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 second test drive, this second detection causes the MIL to light up and O/D OFF indicator lights to blink.