The procedure for reading diagnostic trouble codes (DTCs) is to use a diagnostic scan tool. When reading DTCs, follow the instructions supplied by tool manufacturer.

On on-board diagnostic (EOBD) passenger cars there are 5 options available in the scan tool DTC mode to display the enhanced information available. A description of the new modes, DTC Info and Specific DTC, follows. 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. 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 to search for a specific type of stored DTC information. There are 7 choices. The service manual may instruct the technician to test for DTCs in a certain manner. Always follow published service procedures.

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 will display 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. DTC tests which run and pass will cause that DTC number to be removed from the scan tool screen.

This selection will display all DTCs that have failed during the present ignition cycle.

This selection will display only DTCs that are stored in the ECM's history memory. It will not display type CNL DTCs that have not requested the malfunction indicator lamp (MIL). It will display all type A, B, and E DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, it will display all type C and type D DTCs that have failed within the last 40 warm-up cycles.

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

This selection will display only DTCs that are requesting the MIL. Type C and type D DTCs cannot be displayed using this option. This selection will report type B DTCs only after the MIL has been requested.

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

This mode is used to check the status of individual diagnostic tests by DTC number. This selection can be accessed if a DTC has passed, failed, or both. Many EOBD 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.

The F2 key is used, in this mode, to display a description of the DTC. The YES and NO keys may also be used to display more DTC status information. This selection will only allow entry of DTC numbers that are supported by the vehicle being tested. If an attempt is made to enter 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.

This message display indicates that the last diagnostic test failed for the selected DTC. For type A and type B DTCs, this message will be displayed during subsequent ignition cycles until the test passes or DTCs are cleared. For type C and type D DTCs, this message will clear when the ignition is cycled.

This message display indicates that the DTC has failed at least once within the last 40 warm-up cycles since the last time DTCs were cleared.

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

This message display indicates that the DTC has been stored in memory as a valid fault. A DTC displayed as a History fault may not mean that the fault is no longer present. The history description means that all the conditions necessary for reporting a fault have been met, maybe even currently, and the information was stored in the control module memory.

This message display indicates that the DTC is currently causing the MIL to be turned ON. Remember that only type A, type B, and type C DTCs can request the MIL. The MIL request cannot be used to determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive will require up to three trips during which the diagnostic test passes to turn OFF the MIL.

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

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

This message display indicates that the selected diagnostic test has done the following:

If 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 and further diagnosis is required.

Prior to repairing a vehicle, status information can be used 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.

Notice: Turn the ignition OFF when installing or removing the ECM connectors and disconnecting or reconnecting the power to the ECM (battery cable, ECM pigtail, ECM fuse, jumper cables, etc.) in order to prevent internal ECM damage.

When the ECM sets a diagnostic trouble code (DTC), the malfunction indicator lamp (MIL) lamp will be turned ON only for type A, B, and E, but a DTC will be stored in the ECM's memory for all types of DTC. If the problem is intermittent, the MIL will go out after 10 seconds if the fault is no longer present. The DTC will stay in the ECM's memory until cleared by the scan tool. Removing battery voltage for 10 seconds will clear some stored DTCs. DTCs with type X do not occur in that version

DTCs should be cleared after repairs have been completed. Some diagnostic tables will tell you to clear the codes before using the chart. This allows the ECM to set the DTC while going through the chart, which will help to find the cause of the problem more quickly.