System Operation

PURPOSE
The Engine Control Module (ECM) contains a built-in self diagnostic system by which malfunctions within the engine signal network are detected. By analyzing various signals, The ECM can detect a malfunction and a Diagnostic Trouble Code (DTC) is stored in its memory for help with diagnosis. At the same time the Malfunction Indicator Lamp (MIL) on the combination meter lights up to indicate the ECM has detected a malfunction and stored the DTC. This Diagnostic system is compliant to OBDII regulations in that an OBDII scan tool is used to extract DTC's and that the DTC's follow SAE J-2012 naming guidelines.

OPERATION
The On-Board Diagnostic system operates as follows:

Two Trip Detection
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 test drive, this second detection causes the MIL to light up. The Two Trip repeats the same mode the second test drive, However, the ignition switch must be turned OFF between the 1st trip and the 2nd trip. If the malfunction does not reoccur in 3 trips, the MIL goes off but the diagnostic trouble codes remain recorded in the ECM memory.

Normal Mode / Check Mode
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. Compared to the normal mode, the check mode has an increased sensitivity to detect malfunctions. Also, the same diagnostic items which can be detected in the normal mode can also be detected in the check mode. Most diagnostic trouble codes use two 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 (TOYOTA hand-held tester only).

Freeze Frame Data
Freeze Frame Data records the engine condition when a misfire, fuel trim malfunction, or other malfunction (first malfunction only), is detected. 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.