System Description

DIAGNOSIS SYSTEM




- When troubleshooting OBD II vehicles, the only difference from the usual troubleshooting procedure is that you need to connect an OBD II scan tool (compliant with SAE J1978) or hand-held tester to the vehicle, and read various data output from the vehicle's ECM.

- OBD II regulations require that the vehicle's on-board computer lights up the MIL on the instrument panel when the computer detects a malfunction in the computer itself or in the drive system components which affect the vehicle emissions. In addition to the MIL lighting up when a malfunction is detected, the applicable DTCs prescribed by SAE J2021 are recorded in the ECM memory.

If the malfunction does not reoccur for 3 consecutive drive cycles, the MIL turns off but the DTCs remain recorded in the ECM memory.




- To check the DTCs, connect the hand-held tester or the OBD II scan tool to the DLC3 of the vehicle. The hand-held tester and the OBD II scan tool also enable you to erase the DTCs and check freeze frame data and various forms of engine data (for operation instructions, see the OBD II scan tool's instructions).

- The diagnosis system operates in the normal mode during normal vehicle use, and also has a check mode for technicians to simulate malfunction symptoms and perform troubleshooting. Most DTCs use 2-trip detection logic (*) to prevent erroneous detections. 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) (see step 3).

- 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 cycle, 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 sequential transmission condition when a malfunction (first malfunction only) is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, etc. at the time of the malfunction.