U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This vehicle utilizes the Class 2 communications system. Each bit of information can have one of 2 lengths, long or short. This allows the vehicle wiring to be reduced by the transmission and reception of the multiple signals over a single wire. The messages which are carried on Class 2 data streams are also prioritized. In other words, if 2 messages attempt to establish communications on the data line at the same time, only the message with the 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 sold in the United States.

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

A diagnostic test is a series of steps which has a beginning and an end. The result of which is a pass or fail 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 and key OFF cycle in which all of the enabling criteria for a given diagnostic has been met allowing the diagnostic to run vehicle operation, followed by an engine OFF period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature, such as driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip, such as the catalyst monitor, while others run continuously, such as the misfire and fuel system monitors. If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

A warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen more than 4°C (40°F) from the start-up temperature and reached 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 the functional checks are designed in order to locate a poor circuit or a malfunctioning 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 which are 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 malfunction indicator lamp (MIL) 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 which the technician may not detect if other diagnostics are performed first.

The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:

When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns ON the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:

When the diagnostic executive requests the service lamp to be turned ON or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.

Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.

Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.

When the ignition is turned OFF, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a fuel trim malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal type B diagnostic. The diagnostic executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is captured is called Freeze Frame data. The Freeze Frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the Freeze Frame buffer.

Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the failure records buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:

Freeze Frame data can only be overwritten with the data associated with a misfire or a fuel trim malfunction. The data from these faults take precedence over data that is associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.

In the case of an intermittent fault, the malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When 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:

The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.

The driver may note a change in vehicle performance. In order to allow the VCM to relearn, drive the vehicle at part throttle with moderate acceleration.

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

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

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

1. Review the Freeze Frame and Failure Records data for the DTC which was diagnosed. Record the Freeze Frame and Failure Records data. The Freeze Frame data will only store for an A or B type diagnostic.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the Freeze Frame and Failure Records data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.

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

Use a diagnostic scan tool in order to read the diagnostic trouble codes. Failure to follow this step could result in unnecessary repairs.

In order to clear diagnostic trouble codes (DTCs), use the diagnostic scan tool Clear DTCs or Clear Info function. When clearing DTCs, follow the instructions supplied by the tool manufacturer.

The OBD II vehicles have three options available in the scan tool DTC mode in order to display the enhanced information available. A description of the new modes, the DTC Info, and the Specific DTC follows. After selecting the DTC, the following menu appears:

The following is a brief description of each of the sub menus in the DTC Info and the 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 DTCs in a certain manner. Always follow the published service procedures.

In order to get a complete description of any status, press the Enter key before pressing the desired F-key. For example, pressing Enter, then an F-key will display a definition of the abbreviated scan tool status.

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. The DTC tests which run and pass removes that DTC number 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 history memory of the control module. 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 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 which have failed during the last time that the test ran. The last test may have ran during a previous ignition cycle if the type A or B DTC is displayed. 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. Type C DTCs cannot be displayed by using this option. This selection will report type B DTCs only after the MIL has been requested.

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

This selection displays all of the active and history DTCs that have reported a test failure since the last time the 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 the individual diagnostic tests by the DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the diagnostic executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of the DTC numbers that are supported by the vehicle that is being tested. If an attempt is made to enter the DTC numbers for tests which 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 using the 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 the 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 has 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 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 being experienced. 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 that the selected diagnostic test has the following items:

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 Ign 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.