Diagnosis System

DESCRIPTION
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 LEXUS 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 LEXUS hand-held tester to Data Link Connector 3 on the vehicle. The OBD II scan tool or LEXUS 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. (LEXUS 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 PO3OO P0306) 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 P0300 P0306).
- Fuel trim malfunction (DTC PO170) and EGR (DTC P0401 P0402).
- Misfire (DTC P0300, P0306).