On Diesel Engine equipped vehicles, the Powertrain Control Module (PCM), the Transmission Control Module (TCM) ,if equipped, and the Electronic Brake Control Module (EBCM) are accessed by the Scan Tool through CKT  410 (DK/BLU) or CKT 1921 (DK/BLU) and CKT 409 (LT/GRN) or CKT 1922 (LT/GRN). The scan tool also accesses the PCM through CKT 448 (WHT) or CKT 679 (WHT/BLK).

On gasoline powered vehicles, the PCM is accessed through CKT 547 (ORN/WHT) and CKT 549 (WHT/BLK). The scan tool accesses the EBCM (hydraulic) through CKT 396 (BRN/WHT) on these vehicles and on CKT 799 (TAN/WHT) on diesel powered vehicles.

These communication lines are used to monitor the modules for diagnostic purposes and to check for Diagnostic Trouble Codes (DTCs). Voltage is applied at all times to the DLC through the Circuit Breaker 5. The DLC is permanently grounded through G 105 and G 113.