Back-Up Fuel Mode

WITHOUT DIAGNOSTIC SCAN TOOL
If the diagnostic terminal is grounded with the engine operating, the system will enter the "Field Service" mode. In this mode, the 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.

WITH DIAGNOSTIC SCAN TOOL
Road Test (Open) Mode
On engines that can be monitored in the "Open" mode, certain parameters can be observed without changing the engine operating characteristics The parameters capable of being read vary with engine families. Most scan tools are programmed so that the system will go directly into the DLC mode if the "Open" mode is not available.