DTC will set when the following conditions are present:

WARNING: This page is about a different car, the 1997 GMC Pickup and 1997 Chevrolet Pickup. However, it is still accessible from the selected car via links, so may be relevant.

No VSS DTCs are set.
No TP sensor DTCs are set.
No HO2S DTCs are set.
No misfire DTCs are set.
No MAP sensor DTCs are set.
No fuel trim DTCs are set.
No ECT sensor DTCs are set.
Engine coolant temperature is greater than 185°F (85°C).
Vehicle is in "closed loop" fuel control.
Listed conditions are met for a period of time to ensure a warm catalyst (at least 2 consecutive minutes).
Calculated engine load is steady.
Vehicle speed is steady between 20-70 MPH.
Catalyst monitored HO2S signal activity has been determined to be excessive by VCM.
Listed conditions are met for up to 2 additional minutes after catalyst warm-up criteria has been met.