To comply with OBD II Regulations, the Control Module is equipped with software designed to allow it to monitor vehicle emission control systems and components. Once the ignition is turned on or the engine is started, and certain test conditions are met, the PCM runs a series of monitors to test the emission control systems and components. Test conditions include different inputs such as time since startup, run-time, engine speed and temperature, transaxle gear position, and the engine open or closed loop status. Once the monitor is started, the control module attempts to run it to completion. If a particular monitor fails a test, a code is set and operating conditions at that time are recorded in memory. If the same component or system fails twice in succession, the Malfunction Indicator Lamp (MIL) is activated.
Monitors are divided into 2 types: Main Monitors and Comprehensive Component Monitors.
Certain monitors, in particular the fuel system and misfire monitors, have limitations that are different from any of the other monitors. The first time either of these monitors fail, the MIL is activated and engine conditions at the time of the fault are recorded. In order for the control module to turn off, an MIL related to these 2 monitors, must determine that no faults are present with engine operating conditions similar to when it detected the fault. To qualify, the engine must be operated within a specified speed range, engine load range and temperature range.
A warm-up cycle is considered to be a vehicle operation after the engine has been turned off for a period of time, with the ECT input rising a specified amount and reaching normal operating temperature. When the MIL is turned off because a fault is no longer present, most OBD II codes will be erased after a minimum of 40 warm-up cycles. Misfire and fuel system codes require a minimum of 80 warm-up cycles before they clear.
OBD II Systems use a standardized test connector, called the Data Link Connector (DLC); it is located under the left side of the instrument panel. The DLC is located out of the line of sight of vehicle passengers, but is easily viewable from a kneeling position outside the vehicle. The connector is rectangular in design and contains up to l6 terminals. It has keying features to allow for easy connection. Both the DLC and Scan Tool connectors have latching features that ensure the scan tool will remain properly connected.
Some common uses of the Scan Tool are to identify and clear Diagnostic Trouble Codes (DTCs) and to read control module freeze frame.
The Malfunction Indicator Lamp (MIL) looks similar to the "Check Engine" lamp. However, on OBD II Systems, it is controlled under a strict set of guidelines that dictate when the MIL is illuminated. If any of the control module monitors detect a fault that could impact vehicle emissions, a fault code is set. A One-Trip Monitor requires that a test fail once, a Two-Trip Monitor requires a test fail twice in succession, and a Three-Trip Monitor requires that a test fail three times in succession to activate the MIL.
The MIL is mounted in the instrument panel and has 2 functions: To act as a bulb check at key On and to inform the driver that an emissions fault has occurred.
Once the engine is started, if no faults are detected, the control module should extinguish the MIL after a few seconds. If the MIL remains On or flashes with the engine running, a driveability symptom is present.