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Entering Diagnostic Mode

  1. To obtain a stored trouble code from ECM, the Assembly Line Communication Link (ALCL) is used. The ALCL connector is located in passenger compartment. To enter diagnostic mode, connect diagnostic terminal (terminal "B") to ground (terminal "A") with engine off. See Fig 1 .
    Fig 1: ALCL Connector
    G5361Courtesy of GENERAL MOTORS CORP.
  2. The ECM will first display a code 12, to indicate system is operating. Codes are displayed by flashing either the "CHECK ENGINE" or "SERVICE ENGINE SOON" light. Code 12 consists of a single flash, followed by a short pause, then 2 quick flashes in succession.
  3. Other codes are displayed in a similar manner. Each code will be displayed 3 times. After all codes have been displayed, the ECM will return to flashing code 12. It will flash code 12 until diagnostic terminal ground is removed.
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