System Operation

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 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 (DTC) prescribed by SAE J2012 are recorded in the ECM memory. If the malfunction does not reoccur in 3 trips, the MIL goes off but the DTC remain recorded in the ECM memory.




To check the DTC, 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.)

DTC 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. Compared to the normal mode, the check mode has an increased sensitivity to detect malfunctions. Furthermore, the same diagnostic items which are detected in the normal mode can also be detected in the check mode.
Most DTC use 2 trip detection logic [*] to prevent erroneous detection and ensure thorough malfunction detection.
By switching the ECM to check mode (Toyota hand-held tester only) when troubleshooting, the technician can cause the MIL to light up for a malfunction that is only detected once or momentarily.

[*] 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 - P0306) or fuel trim malfunction (DTC P0171, P0172), 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 DTC according to the following priorities.
(1) DTC other than fuel trim malfunction (DTC P0171, P0172), EGR (DTC P0401, P0402), and misfire (DTC P0300 - P0306).
(2) Fuel trim malfunction (DTC P0171, P0172) and EGR (DTC P0401, P0402).
(3) Misfire (DTC P0300 thru P0306).

DLC3 Inspection

DLC3 Terminal ID:

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.

(1) If communication is normal when the tool is connected to another vehicle, inspect DLC3 on the original vehicle
(2) 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 Mode
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 5° BTDC, without relation to the engine speed.

NOTE: If the engine is controlled by the back-up function, the MIL lights up to warn the driver of the malfunction but the DTC is not output.