Entering Or Exiting Diagnostic Mode
- With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig 1. The Diesel Electronic Control (DEC) system will enter the diagnostic mode.
- In this mode, the ECM will display Code 12, by flashing the SERVICE ENGINE SOON light. Code 12 consists of one (1) flash, followed by a short pause, then 2 flashes in quick succession.
- 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.
Courtesy of GENERAL MOTORS CORP.