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Home >> GMC >> 1995 >> Forward Control 4.3 Z, Standard >> Repair and Diagnosis (Single Page) >> Transmission >> Automatic Trans >> Automatic Transmission Diagnosis - 4L80-E >> Electronic Self-Diagnostics >> Diagnostic Procedure (Diesel Engines) >> Entering Or Exiting Diagnostic Mode

Entering Or Exiting Diagnostic Mode

  1. With ignition on and engine off, connect a jumper wire between Data Link Connector (DLC) terminal "B" (diagnostic terminal) and terminal "A" (ground). See Figure. Diesel Electronic Control (DEC)/electronically-controlled transmission systems will enter diagnostic mode.
  2. In this mode, PCM/TCM will display DTC 12 by flashing the MIL once, followed by a short pause, then 2 flashes in quick succession. DTC 12 will be displayed 3 times. If no other codes are stored, DTC 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from DLC.
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