Entering Service Mode

After trouble codes have been displayed, the Service Mode can be used to perform several tests on different systems, one at a time. Upon completion of code displays, a specific system may be selected for testing. To operate diagnostic service modes, continue as follows.

1. Turn ignition switch to ON position. Enter diagnostics. See Entering Diagnostic Mode . See Fig 1. After trouble codes have all been displayed or by-passed (by pressing "LO" button on the CCP), the diagnostic service mode will be entered.
2. ECM will be the first available service mode system displayed, followed by BCM, Lighting and Anti-Lock Brakes. To select a system to test, any of the following actions may be taken.
   • Depressing the "OFF" button on CCP will stop system selection process and return display to beginning of trouble code sequence.
   • Depressing the "LO" button on CCP will display next available system selection. This allows display to be stepped through all system choices. This list of systems can be repeated following end of system list.
   • Depressing "HI" button on CCP will select displayed system for testing.
3. Once a system is selected, the first available test type will be displayed (i.e. ECM, DATA?). While selecting a specific test type, any of the following actions may be taken to control the display.
   • Depressing the "OFF" button on CCP will exit test type selection process and return display to next available system selection.
   • Depressing the "LO" button on CCP will display next available test type for selected system. This allows display to be stepped through all available test type choices. This list of test types can be repeated following display of last test type.
   • Depress the "HI" button on CCP will select the displayed test type. At this point the first of several specific tests will appear.
4. Selection of "DATA?", "INPUTS?", "OUTPUTS?", or "OVERRIDE?" test types will result in first available test, being displayed. If dashes ever appear, this test is not allowed with engine running. The 4 characters of the display will contain a test code to identify selection. The first 2 characters are letters which identify system and test type(i.e. ED for ECM DATA). The last 2 characters numerically identify the test(i.e. ED01 for Throttle Position). To select a specific test, any of the following actions may be taken. To control display choices:
   • Depressing the, "OFF" button on CCP will exit test selection process and return display to next available test type for select system.
   • Depressing the "LO" button on CCP will display next smaller test number for selected test type. If this button is pressed with lowest test number displayed, the highest test number will then appear.
   • Depressing the "HI" button on CCP will display next larger test number for the selected test type. If this button is pressed with highest test number displayed, the lowest test number will then appear.
5. Selection of "CLEAR CODES?" test type will result in message "CODES CLEAR" being displayed along with selected system name. This message will appear for 3 seconds to indicate that all stored trouble codes have been erased from that system's memory. After 3 seconds the display will automatically return to next available test type for selected system.
NOTE: For details of the type of displays available, see DATA DISPLAYS .
6. Selection of "SNAPSHOT?" test type will result, in message "SNAPSHOT TAKEN" being displayed with selected system name proceeding it. This message will appear for 3 seconds to indicate that all system data and inputs have been stored in memory. After 3 seconds the display will automatically proceed to the first available snapshot test type. To select a snapshot test type, any of the following actions may be takento control the display.
   • Depressing the "OFF" button on CCP will exit test type selection process and return display to next available system selection.
   • Depressing the "LO" button will display the next available snapshot test type. This allows the display to be stepped through all available choices. This list of snapshot test types can be repeated following display of last choice.
   • Depressing the "HI" button with "SNAPDATA?" or "SNAP INPUTS?" displayed will select that test type. At this point the display is controlled as it would be for non-snapshot data and input displays. All values and status information, however represent memorized vehicle conditions.
   • Depressing the "HI" button with "SNAPSHOT?" displayed will again display "SNAPSHOT TAKEN" message to indicate that new information has been stored in memory. Access to this information is obtained the same as previously described.

Fig 1: Diagnostic Service Mode Testing Sequence

Courtesy of GENERAL MOTORS CORP.