DTC will set when the following conditions are present:

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 1997 GMC Savana Special, 1997 GMC Savana Camper Special, 1997 GMC Savana, 1997 Chevrolet RV Cutaway, and 1997 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.

Vehicle speed is at least one MPH.
Engine speed is less than 5800 RPM.
Engine load is not greater than 89.9 percent.
DTC P0300 is set and requesting MIL on.
Rough road indicator not being sent by EBCM.