System Operation
DESCRIPTIONWhen 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 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 malfunction indicator lamp lighting up when a malfunction is detected, the applicable diagnostic trouble codes prescribed by SAE J2O12 are recorded in the engine control module memory.
If the malfunction does not recur in 3 trips, the malfunction indicator lamp goes off but the diagnostic trouble codes remain recorded in the engine control module memory.
To check the diagnostic trouble codes, connect the OBD-II scan tool or TOYOTA hand-held tester to Data Link Connector 3 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.)
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 engine control module to check mode when troubleshooting, the technician can cause the malfunction indicator lamp to light up for a malfunction that is only detected once or momentarily. (TOYOTA hand-held tester only)
2 trip detection logic
When a logic malfunction is first detected, the malfunction is temporarily stored in the engine control module memory. If the same malfunction is detected again during the second drive test, this second detection causes the malfunction indicator lamp 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 P0304) 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.