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Entering & Exiting Diagnostic Mode

NOTE: The Assembly Line Data Link (ALDL) may also be referred to as the Data Link Connector (DLC) in flow charts.
  1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (test terminal) and terminal "A" (ground). See Fig 1. This places control module into diagnostic mode.
  2. In this mode, control module will display Code 12 by flashing SERVICE ENGINE SOON (SES) light. Code 12 is identified by SES light flashing once, followed by a short pause, then 2 flashes in quick succession.
  3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until test terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Fig 1: Identifying ALDL Connector Terminals
G90B01199Courtesy of GENERAL MOTORS CORP.