Description of On-Board Diagnostics

DESCRIPTION
When troubleshooting OBDII vehicles, the only difference from the usual troubleshooting procedure is that you connect the OBDII scan tool complying with SAE J1978 or LEXUS hand-held tester to the vehicle. and read off various data output from the vehicle's ECM.
OBDII 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 DTCs prescribed by SAE J 2012 are recorded in the ECM memory.
If the malfunction does not reoccur in 3 trips, the MIL goes off but the DTCs remain recorded in the ECM memory.
To check the DTCs, connect the OBDII scan tool or LEXUS hand-held tester to DLC3 on the vehicle. The OBDII scan tool or LEXUS hand-held tester also enables you to erase the DTCs and check freezed frame data and various forms of engine data.

(For operating instructions, see the OBDII scan tool's instruction book.)

DTCs 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 DTCs 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 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.