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 diagnostic mode.
- In this mode, ECM will display Code 12 by flashing the SERVICE ENGINE SOON light once, 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 diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from ALDL connector.
Courtesy of GENERAL MOTORS CORP.