Field Service Mode Description

Duty Check Datalink Connector:

FIELD SERVICE MODE
Ignition Timing
If the test switch terminal in the underhood Duty Check Data Link Connector (DLC) is grounded, the Engine Control Module (ECM) sets ignition timing to its initial (non-corrected) setting.

Duty Cycle
If both the diagnostic terminal and the test switch terminal are grounded simultaneously, the ECM outputs the air/fuel duty cycle through the duty check terminal of the Duty Check DLC.