The U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows the vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on the Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing the data from any make or model vehicle that is sold in the United States.

The Diagnostic Executive is a unique segment of software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:

A diagnostic test is a series of steps that has a beginning and an end. The result of which is a pass or a fail which is reported to the Diagnostic Executive. When a diagnostic test reports a pass result, the Diagnostic Executive records the following data:

When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:

The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as a key ON cycle in which all of the enabling criteria for a given diagnostic have been met, allowing the diagnostic to run during vehicle operation, followed by a key OFF period of sufficient duration to allow the PCM to go to 'sleep'. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip (e.g. catalyst monitor), while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions for a particular diagnostic test are not met during an ignition cycle, the tests may be incomplete or may not run.

A warm-up cycle consists of an engine start-up and a vehicle operation in which the coolant temperature rises to at least 22°C (40°F) from the start-up temperature and reaches a minimum engine coolant temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.

The diagnostic tables and functional checks are designed to locate an improperly functioning circuit or component through a process of logical decisions. The tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the Malfunction Indicator Lamp (MIL) will illuminate on some applications.

The MIL (Malfunction Indicator Lamp) is on the instrument panel. The MIL has the following functions:

When the MIL remains on while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults that the technician may not detect if other diagnostics are performed first.

In the case of an intermittent malfunction, the Malfunction Indicator Lamp (MIL) may illuminate, but will turn off after 3 trips. However, the corresponding diagnostic trouble code will store in the memory. When an unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

The provision for communicating with the control module is a Data Link Connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:

Some vehicles allow the reprogramming of the control module without removing the module from the vehicle. This provides a flexible and a cost-effective method of making changes in the software and the calibrations.

Refer to the latest Techline information on the reprogramming or the flashing procedures.

Verification of the vehicle repair will be more comprehensive for the vehicles with the OBD II system diagnostics. Following a repair, the technician should perform the following steps:

1. Review the fail records and the freeze frame data for the DTC that was diagnosed. Record the fail records or freeze frame data. The freeze frame data will only store for an A or B type diagnostic and only if the MIL has illuminated.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the fail records or the freeze frame data.
4. Monitor the DTC status information for the specific DTC that has been diagnosed until the diagnostic test associated with that DTC runs.

Following these steps is very important in verifying the repairs on the OBD II systems. Failure to follow these steps could result in unnecessary repairs.

A diagnostic scan tool is used to read the diagnostic trouble codes . When reading DTCs, follow all steps carefully to avoid unnecessary repairs.

On the OBD II vehicles there are three options available in the scan tool DTC mode to display the enhanced information available. the following is a description of the new modes: DTC Info and Specific DTC. After selecting DTC, the following menu appears on the scan tool:

The following is a brief description of each of the sub menus in DTC Info and Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.

Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for the DTCs in a certain manner. Always follow the published service procedures.

This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. When DTC tests run and pass, that DTC number is removed from the scan tool screen.

This selection displays all of the DTCs that have failed during the present ignition cycle.

This selection displays only the DTCs that are stored to the control module history memory. The history memory will not display the type B DTCs that have not requested the MIL. The history memory will display all of the type A and type B DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.

This selection displays only the DTCs that failed the last time the test ran. If a type A or B DTC is displayed, the last test may have run during a previous ignition cycle . For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

This selection displays only the DTCs that are requesting the MIL. The type C DTCs cannot be displayed by using this option. This selection will report any type B DTCs only after the MIL has been requested.

This option displays up to 33 DTCs that have not run since DTCs were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.

This selection displays all active and history DTCs that have reported a test failure since the last time DTCs were cleared. The DTCs that last failed over 40 warm-up cycles before this option is selected will not be displayed

This mode is used in order to check the status of individual diagnostic tests by DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because the Diagnostic Executive monitors extensive amounts of information about each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of DTC numbers that are supported by the vehicle being tested. If an attempt is made to enter DTC numbers for tests that the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to usingthe DTC trigger option in the Snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

For type A and B DTCs, this message will display during subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.

This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.

This message displayed indicates that the diagnostic test failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.

This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a History fault does not necessarily mean that the fault is no longer present. The history description means that all of the conditions necessary for reporting a fault have been met.

This message displayed indicates that the DTC is currently causing the MIL to turn on. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently present. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn off the MIL.

This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status (passing or failing) is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.

This message displayed indicates that the selected diagnostic test has not run this ignition cycle.

This message displayed indicates the following about the selected diagnostic:

Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.

If the indicated status of the vehicle is Failed This Ignition after a repair verification, then the repair is incomplete. A further diagnosis is required.

Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.