Diagnosis System

DIAGNOSIS SYSTEM

a. Description
- When troubleshooting OBD II vehicles, the only difference from the usual troubleshooting procedure is that you need to connect the vehicle to the hand-held tester or OBD II scan tool complying with SAE J1978, 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 Light (MIL) on the instrument panel when the computer detects a malfunction in the emission control system/components or in the powertrain control components which affect vehicle emissions, or a malfunction in the computer. In addition to the MIL lighting up when a malfunction is detected, the applicable Diagnostic Trouble Code (DTC's) prescribed by SAE J2012 are recorded in the ECM memory.

If the malfunction does not reoccur in the 3 consecutive trips, the MIL goes off automatically but the DTC's remain in the ECM memory.




- To check the DTC's, connect the hand-held tester or OBD II scan tool to the Data Link Connector 3 (DLC3) of the vehicle. The hand-held tester or OBD II scan tool also enables you to erase the DTC's and check freeze frame data and various forms of engine data (For operating instructions, see the OBD II scan tool's instruction book.). DTC's 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 a manufacturer within the prescribed limits (See the DTC chart).

- The diagnosis system operates in the normal mode during normal vehicle use. It also has a check mode for technicians to simulate malfunction symptoms and troubleshoot. Most DTC's use 2 trip detection logic* to prevent erroneous detection and ensure the malfunction detection. By switching the ECM to the check mode when troubleshooting, a technician can cause the MIL to light up for a malfunction that is only detected once or momentarily (hand-held tester only) (See step 2).

- *2 trip detection logic:
When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory. (1st trip)

If the same malfunction is detected again during the second drive test, this second detection causes the MIL to light up (2nd trip) (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's P0300 - P0304) or fuel trim malfunction (DTC's P0171 and P0172) or other malfunction (first malfunction only), is detected. Freeze frame data records the engine conditions (fuel system, calculated 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 was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.

Priorities of troubleshooting:
If troubleshooting priorities for multiple DTC's are given in the applicable DTC chart, these should be followed.
If no instructions are given, troubleshoot DTC's according to the following priorities
1. DTC's other than fuel trim malfunction (DTC's P0171 and P0172), and misfire (DTC's P0300 - P0304).
2. Fuel trim malfunction (DTC's P0171 and P0172).
3. Misfire (DTC's P0300 - P0304).







b. Check the DLC3.
The vehicle's ECM uses ISO 9141-2 for communication.
The terminal arrangement of the DLC3 complies with SAE J1962 and matches the ISO 9141-2 format.

HINT: If your display shows UNABLE TO CONNECT TO VEHICLE when you have connected the cable of the hand-held tester or OBD II scan tool to the DLC3, turned the ignition switch ON and operated the scan tool, there is a problem on the vehicle side or tool side.
- If the communication is normal when the tool is connected to another vehicle, inspect the DLC3 of the original vehicle.
- If the communication is still impossible 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.