Field Service Mode Description

FIELD SERVICE MODE DESCRIPTION
If the diagnostic terminal is grounded with the engine operating, the system will enter the "Field Service" mode. In this mode, the Malfunction Indicator Lamp (MIL) (Service Engine Soon) will indicate whether the system is in "Open Loop" or "Closed Loop."
In "Open Loop" the malfunction indicator lamp 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 in the control module and the "Closed Loop" timer is bypassed.