Each diagnostic trouble code (DTC) is directly related to a diagnostic test. The diagnostic management system sets DTCs based on the failure of the tests during a trip or trips. Certain tests must fail 2 consecutive trips before the DTC is set. The following are the 3 types of DTCs and the characteristics of those codes:

Type A

Type B

Type Cnl

Type E

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

Important: Do not clear DTCs unless directed to do so by the service information provided for each diagnostic procedure. When DTCs are cleared, the Freeze Frame and Failure Record data which may help diagnose an intermittent fault will also be erased from memory. If the fault that caused the DTC to be stored into memory has been corrected, the diagnostic executive will begin to count the warm-up cycles with no further faults detected, the DTC will automatically be cleared from the engine control module (ECM) memory.

To clear DTCs, use the diagnostic scan tool.

It can not clear DTCs without the diagnostic scan tool. So you must use the diagnostic scan tool.

Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the diagnostic executive will command the MIL to flash as a rate of once per seconds during the time that the catalyst damaging misfire condition is present.

Fuel trim and misfire are special cases of Type B diagnostics. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperature (ECT) 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 malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal Type B diagnostic, and does not use the stored conditions. However, if a 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:

On Euro 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 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 have 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 and type E 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 the 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 and type B 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 3 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.