Clark... there can be many reasons why an engine is running rich. As most of today's engines are computer controlled, and meter fuel based on data they receive from sensors (MAF - measuring air inflow; TPS - measuring throttle position; O2S - measuring residual oxygen for fine tuning the mixture once the car goes into "closed loop" operation (eg once the entire system is warmed up and the computer starts controlling fuel feed rate based on actual data instead of pre set parameters), and a high candidate; CPS - crankshaft position sensor, and there are others. When they diagnose the car, check the functionality of your 02 sensors. They can actually watch the output voltage vary on them in an OBDII tester. It should (on the pre-cat sensor(s) fluctuate between 100 and 900 millivolts, crossing over the .445 V trigger point frequently. As for a warm engine, that always help emissions checks, but as for the O2S's, I 'm positive the 2000 model year used heated sensors, which helps them become operational sooner and to remain operational even whlie your engine is idling (they need 650F plus to be operational). While there are non sensor things that neeed to be considered, the usual course is to check air filter, and then sensors. From there the list expands to other non-computer controlled items. Good luck.
