Did you replace the cat with an oem type one or a muffler shop cat? I know some muffler shop cats are known to be 2 year cats.
Taken from www.smogtips.com
3. Defective Catalytic Converter (CAT)
Some vehicle manufactures have designed their cars to operate without EGR valves. Non-EGR equipped vehicles rely heavily on the Catalytic Converter to assist in the reduction of NO. These vehicles have tendencies to develop CAT problems sooner then those which are equipped. If you own a non-EGR equipped vehicle, and have failed the emissions test for high NOx, pay close attention to the Catalytic Converter.