Field Service Mode Description

If the diagnostic terminal is grounded with the engine running, the system will enter the Field Service mode.
In "Open Loop" the Malfunction Indicator Lamp (MIL) (Service Engine Soon) flashes two and one-half times per second.
In "Closed Loop," the light flashes once per second. Also, in "Closed Loop," the light will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.
While the system is in Field Service Mode, new codes cannot be stored and the "Closed Loop" timer is bypassed.