Without Scan Tool

ALDL Connector:

Diagnostic Mode:
With the key "ON" and engine "OFF", jumper ALDL terminal "B" to "A". The diesel electronic control system will enter the diagnostic mode code system in this mode, the ECM will:

^ Display a code 12, by flashing the "Service Engine Soon" light (indicating the system is operating). A code 12 consists of one flash, followed by a short pause, then two flashes in quick succession. This code will be flashed three times. If no other codes are stored, code 12 will continue to flash until the diagnostic terminal is ungrounded. If code 12 does not display, refer to Engine Control Module (ECM) Check.

A "hard" code is one which is present when you are working on the vehicle and the condition still exists while working on the vehicle. The chart with the stored code number will lead you to the cause of the problem.

An "intermittent" code is one which does not reset itself, and is not present while you are working on the vehicle. This is often caused by a loose connection.