Description of On-Board Diagnostics
DESCRIPTIONWhen 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 Engine Control Module (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 transmission control section, the MIL lights up and the PWR indicator light blinks. 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 only occurs in 3 trips, the MIL goes off but the DTCs remain recorded in the ECM memory. The PWR indicator light goes off as soon as the trouble is no longer detected.
DLC3 Is Used As The Data Link Connector For ECT Malfunctions:
To check the DTCs, connect the OBD II scan tool or LEXUS hand - held tester to DLC3 on the vehicle. The OBD II scan tool or LEXUS hand-held tester also enables you to erase the DTCs and check freeze frame data and various forms of engine data.
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. (See DTC chart)
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 [1] to prevent erroneous detection. By switching the ECM to check mode when troubleshooting, the technician can cause the MIL to light up and PWR indicator lights to blink for a malfunction that is only detected once or momentarily. (LEXUS hand-held tester)
[1] 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 PWR indicator lights to blink.