The United States federal regulations require that all automobile manufacturers utilize a common serial data communications system. This system is called the Class 2 communications system. The Class 2 communications system allows multiple signals, or messages to be carried on a single serial data circuit.

The Diagnostic Executive is the name given to a portion of the of the control module programming stored in the EEPROM. The functions of the Diagnostic Executive are as follows:

When the test enabling conditions for a particular test are met, the test will continue to run until one of the following occur:

An ignition cycle consist of the following steps:

1. Start the engine.

Some diagnostic tests require a warm-up cycle in order to run.

2. Operate the vehicle within the conditions for running the particular diagnostic test. The conditions required to run each test vary and may require the following in order to run:

•	A particular style of driving

•	A certain amount of time

•	A certain outside air temperature

3. Turn the ignition OFF.

Some diagnostic tests will only run one time during an ignition cycle. Some diagnostic test run continuously. Some of the diagnostic test may start to run, and stop . Some diagnostic tests may not run within the ignition cycle.

A warm-up cycle consist of the following:

Certain diagnostic test will not run if these conditions are not met during an ignition cycle.

If the control module turns the MIL ON because a DTC is set, the following information is stored in the scan tool and displayed in the Freeze Frame/Failure Records data list.

This information will remain stored in the control module until the following occurs:

Use a scan tool in order to read Diagnostic Trouble Codes (DTC)s. Failure to follow this procedure will result in unnecessary repairs.

Diagnostic Trouble Codes (DTC)s will be cleared by one of the following ways :

Notice: Turn off the ignition key when disconnecting or reconnecting battery power in order to prevent system damage.

Important: Disconnecting the negative battery will result in the loss of other on-board memory data, such as preset radio tuning.

There are 3 types of DTCs. They are as follows:

The Malfunction Indicator Lamp (MIL) is located at the instrument panel. The MIL (Service Engine Soon) has the following functions:

Important: The MIL is turned ON for the above conditions only. If the engine stops running with the ignition switch in the Run or the ON position the MIL is not turned ON.

Perform the Powertrain On-Board Diagnostic (OBD) System Check if the following conditions exist:

If a malfunction is intermittent , the MIL (Service Engine Soon) may turn ON and then turn OFF after 3 ignition cycles. However, the VCM will store the corresponding DTC and certain engine operating data present at the time the malfunction is detected.

The Data Link Connector (DLC) is located under the instrument panel. The DLC is the provision for connecting the scan tool to the serial data communications circuit in order to perform the following:

The Engine Controls Diagnostic Information and Procedures consist of diagnostic tables and functional checks that are developed in order to locate a faulty circuit, component, or system through a logical process of elimination. The diagnostic tables are prepared with assumption that the vehicle functioned correctly at the time of assembly and that there are not multiple malfunctions present.

Following a repair, the technician is to verify that the malfunction is repaired. Verify the repair by performing the following steps:

1. Review and record the Freeze Frame/Failure Records data for the DTC which was diagnosed. The control module will store Freeze Frame/Failure Records data for type A or a type B DTC only, and only then if the MIL has illuminated.
2. Clear the DTCs.
3. Operate the vehicle within the conditions recorded in the Freeze Frame/Failure Records data.
4. Monitor the DTC Status data for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.