On-Board Diagnostic System
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 TOYOTA 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 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 TOYOTA hand-held tester to Data Link Connector 3 (DLC3) on the vehicle. The OBD II scan tool or TOYOTA hand-held tester also enables you to erase the diagnostic trouble codes and check freeze frame data and various forms of engine data. (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. It also has a check mode for technicians to simulate malfunction symptoms and troubleshoot. Most diagnostic trouble codes 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. (TOYOTA hand-held tester only)
*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. The 2 trip repeats the same mode a 2nd time. (However, the IG 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-P0304) 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.
- Diagnostic trouble codes other than fuel trim malfunction (DTC P0170), EGR (DTC P0401, P0402), and misfire (DTC PO3OO - P0304).
- Fuel trim malfunction (DTC PO170) and EGR (DTC P0401, P0402).
- Misfire (DTC P0300 P0304).
DATA LINK CONNECTOR 3 INSPECTION:
The vehicle's ECM uses V.P.W. (Variable Pulse Width) for communication to comply with SAE J1850. 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 OBD II scan tool or TOYOTA 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.