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Entering & Exiting Diagnostic Mode

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  1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig 1. This places the control unit into diagnostic mode.
  2. In this mode, the control unit will display Code 12 by flashing the SERVICE ENGINE SOON (SES) light. Code 12 is identified by the 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 the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Fig 1: Identifying ALDL Connector Terminals
G120880Courtesy of GENERAL MOTORS CORP.
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