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.