Diagnosis System

DIAGNOSIS SYSTEM

- When troubleshooting OBD II vehicles, the only difference from the usual troubleshooting procedure is that you need to connect the vehicle to the OBD II scan tool complying with SAE J1978 or 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 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) prescribed by SAE J2012 is recorded in the ECM memory.

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

- To check the DTCs, connect the OBD II scan tool or hand-held tester to the Data Link Connector 3 (DLC3) on the vehicle. The OBD II scan tool or hand-held tester also enables you to erase the DTCs and check freeze frame data and various forms of engine data (For operating instructions, see the OBD II scan tool's instruction book.). 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 the normal mode during the normal vehicle use. It also has a check mode for technicians to simulate malfunction symptoms and troubleshoot. Most DTCs use 2 trip detection logic* to prevent erroneous detection, which ensures the malfunction detection. By switching the ECM to the check mode when troubleshooting, the 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 2 trip and 2nd 2 trip.).

- Freeze frame data:
Freeze frame data records the engine condition when a misfire (DTC P0300 - P0304) or fuel trim malfunction (DTC 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 a 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 for troubleshooting:
If troubleshooting priorities for multiple DTCs are given in the applicable DTC chart, these should be followed.

If no instructions are given, troubleshoot the DTCs according to the priorities.
1. DTCs other than fuel trim malfunction (DTCs P0171 and P0172) and misfire (DTCs P0300 - P0304).
2. Fuel trim malfunction (DTCs P0171 and P0172).
3. Misfire (DTCs P0300 - P0304).