Class 2 Serial Data

The U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors 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 multiple signals over a single wire. The messages carried on the 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 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

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:

On Board Diagnostic Tests

A diagnostic test is a series of steps with a beginning and an end. Diagnostic tests result in 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:

Trip

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 has been met, allowing the diagnostic to run during vehicle operation. This is followed by a key OFF period of sufficient duration to allow the PCM to power down. 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 (e.g., catalyst monitor), while others run continuously (e.g., misfire and fuel system monitors). If the proper enabling conditions are not met during a given ignition cycle, some tests may not be completed, while other tests may not run.

Warm Up Cycle

A warm-up cycle consists of an engine start-up and a vehicle operation of sufficient duration for the coolant temperature to rise higher than 22°C (40°F) from the start-up temperature and reach a minimum temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information

The diagnostic tables and functional checks are designed to locate a poor 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 MIL will illuminate on some applications.

Malfunction Indicator Lamp (MIL)

The MIL is on the instrument panel. The MIL has the following functions:

When the MIL remains on with the engine running, or when a malfunction is suspected due to a driveability or emissions problem, perform the On-Board Diagnostic (OBD) System Check. The procedures for this check is provided in Engine Controls. This check exposes faults that the technician may not detect if other diagnostics are performed first.

Intermittent Malfunction Indicator Lamp (MIL)

In the case of an intermittent malfunction, the MIL may illuminate, but the MIL will turn off after 3 trips. The corresponding diagnostic trouble code will be stored in the memory, however. When an unexpected diagnostic trouble code appears, check for an intermittent malfunction.

Data Link Connector (DLC)

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:

Reprogramming (Flashing) the Control Module

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.

Verifying Vehicle Repair

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

1. Review the Failure records and the Freeze Frame data for the DTC that was diagnosed. Record the Failure 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 Failure Records or the Freeze Frame data.
4. Monitor the DTC status information for the specific DTC that was diagnosed until the diagnostic test associated with that DTC runs.

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

Reading Diagnostic Trouble Codes

Use a diagnostic scan tool to read the diagnostic trouble codes. Failure to follow the appropriate steps when reading DTCs could result in unnecessary repairs.

DTC Modes

There are 4 options available on OBD II vehicles in the scan tool DTC mode for displaying the enhanced information available. After selecting DTC, the following menu appears:

The following is a brief description of each of the sub menus in DTC Info and Specific DTC.

DTC Info

When the scan tool displays a DTC, the status of the DTC is also displayed. The following DTC information will be indicated only when it applies to the DTC that is set.

Fail This Ign. (Fail This Ignition): Indicates that this DTC failed during the present ignition cycle.

Last Test Fail: Indicates that this DTC failed the last time the test ran. The last test may have run during a previous ignition cycle if an A or B type DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

MIL Request: Indicates that this DTC is currently requesting the MIL. This selection will report type B DTCs only when they have requested the MIL (failed twice).

Test Fail SCC (Test Failed Since Code Clear): Indicates that this DTC has reported a failure since the last time the DTCs were cleared.

History: Indicates that the DTC is stored in the PCM History memory. Type B DTCs will not appear in History until they have requested the MIL (failed twice). History will be displayed for all type A DTCs and type B DTCs (which have requested the MIL) that have failed within the last 40 warm-up cycles. Type C DTCs that have failed within the last 40 warm-up cycles will also appear in History.

Not Run SCC (Not Run Since Code Clear): DTCs will be listed in this category if the diagnostic has not run since DTCs were last cleared. This status is not included with the DTC display since the DTC can not be set if the diagnostic has not run. This information is displayed when DTC Info is requested using the scan tool.

Specific DTC Mode: Used in order to check the status of individual diagnostic tests by DTC number. This selection can be accessed whether 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. 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 using the DTC trigger option in the Snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

Freeze Frame Data

Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data 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 as Freeze Frame data. A subsequent failure will not update the recorded information.

The Freeze Frame data will not be erased unless the associated history DTC is cleared.

Failure Records Data

When a diagnostic test fails, the PCM stores the information as Failure Records data. Unlike Freeze Frame data, Failure Records are stored for multiple DTCs or non-emission related DTCs, that is, DTCs that do not illuminate the MIL. The Failure Records information is updated the first time the test fails during each ignition cycle.

Freeze Frame and Failure Records data may be retrieved with the scan tool. If more than one DTC is set, review the odometer or the engine run time data that is located in the Freeze Frame, Failure Records information in order to determine the most current failure. The stored information will look like normal PCM data.

Clearing Diagnostic Trouble Codes

Use a scan tool to clear DTCs from the PCM memory. Do not disconnect the vehicle battery to clear the PCM memory. Do not disconnect the PCM connectors solely for clearing DTCs. This will not clear the DTCs, unnecessarily disturbs the connections, and may introduce a new problem. Before clearing any DTCs the scan tool has the capability to save any data stored with the DTCs in order to display that data at a later time. Capture the DTC info before beginning diagnosis. Do not clear any DTCs until you are instructed to do so within a diagnostic procedure.

Many PCM DTCs have complex test and failure conditions. Therefore, simply clearing DTCs and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after it is complete, you must look up the test conditions and duplicate those conditions. If the DTC runs and passes the problem is fixed.

Capturing DTC Info (Capture Info)

Selecting this option on the scan tool allows the technician to record the Freeze Frame and Failure Records that may be stored in the PCM memory. Capture DTC info before beginning diagnosis. This is a step in the OBD System Check. At the end of the diagnostic tables, you are instructed to clear DTCs, verify that the repair was successful and then to review captured information. The captured information will contain any additional DTCs and related data that was stored originally if multiple DTCs were stored.

PCM Snapshot Using A Scan Tool

The scan tool can be set up to take a snapshot of the parameters available via serial data. The Snapshot function records live data over a period of time. The recorded data can be played back and analyzed. The scan tool can also graph parameters singly or in combinations of parameters for comparison. The snapshot can be triggered manually at the time a symptom is noticed, or set up in advance to trigger when a DTC sets. An abnormal value captured in the recorded data may point to a system or component that needs to be investigated further. The snapshot will remain in the scan tool memory even after the tool is disconnected from it's power source. Refer to the scan tool user instructions for more information on the Snapshot function.