System Operation

TROUBLESHOOTING OBD II SYSTEMS
When troubleshooting OBDII 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).

Malfunction Indicator Light:

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 prescribed by SAE J2O12 are recorded in the ECM memory.

If the malfunction does not reoccur in 3 trips, the MIL goes off but the diagnostic trouble codes remain recorded in the ECM memory.




To check the diagnostic trouble codes, 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 diagnostic trouble codes and check freeze frame data and various forms of engine data.

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. It also has a Check Mode for technicians to simulate malfunction symptoms and troubleshoot. Most diagnostic trouble codes use 2-trip detection logic [1] 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)

[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 drive test, this second detection causes the MIL to light up.
NOTE: The 2 trip repeats the same mode a 2nd time. However, the ignition switch must be turned OFF between the 1st trip and 2nd trip.

Freeze Frame Data
Freeze frame data records the engine condition when a misfire (DTC P0300 P0308) or fuel trim malfunction (DTC PO170) or other malfunction (first malfunction only), is detected. Because freeze frame data records the engine conditions (fuel system, calculator load, engine coolant temperature, fuel trim, engine speed, vehicle speed, etc.) when the malfunction is detected, when troubleshooting it is useful for determining whether the vehicle was running or stopped, the engine warmed up or not, the air-fuel ratio lean or rich, etc. at the time of the malfunction.

Priorities for Troubleshooting
If troubleshooting priorities for multiple diagnostic codes are given in the applicable diagnostic chart, these should be followed. If no instructions are given troubleshoot diagnostic trouble codes according to the following priorities.
1. Diagnostic trouble codes other than fuel trim malfunction (DTC P0170), Exhaust Gas Recirculation (EGR) (DTC P0401, P0402, P0403), and misfire (DTC P0300 P0308).
2. Fuel trim malfunction (DTC PO170), and EGR (DTC P0401, P0402, P0403).
3. Misfire (DTC P0300 P0308).

DATA LINK CONNECTOR (DLC3)

DLC3 Terminals:

The vehicle's ECM uses V.P.W. (Variable Pulse Width) for communication to comply with SAE J185O. The terminal arrangement of DLC3 complies with SAE J1962 and matches the V.P.W. format.

HINT: If your display shows "UNABLE TO CONNECT TO VEHICLE" when you have connected the cable of the OBDII scan tool or LEXUS hand-held tester to DLC3, turned the ignition switch ON and operated the scan tool, there is a problem on the vehicle side or tool side.
- If communication is normal when the tool is connected to another vehicle, inspect DLC3 on the original vehicle.
- If communication is still not possible when the tool is connected to another vehicle, the problem is probably in the tool itself, so consult the Service Department listed in the tool's instruction manual.

FAIL-SAFE FUNCTION
If any of the following codes is recorded, the ECM enters fail-safe mode.






BACK.UP FUNCTION
If there is trouble with the program in the ECM and ignition signals (IGT) are not output from the microcomputer, the ECM controls fuel injection and ignition timing at predetermined levels as a back-up function to make it possible to continue to operate the vehicle. Furthermore, the injection duration is calculated from the starting signal (STA) and the closed throttle position switch signal (IDL). Also, the ignition timing is fixed at 50 BTDC, without relation to the engine speed.
HINT: If the engine is controlled by the back-up function, the MIL lights up to warn the driver of the malfunction but the diagnostic trouble code is not output.