Field Service Mode Description

RH Rear Of Engine Compartment:

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.