Sorry to bring this up from the dead, but in my search for cats for my 2001 V8 I came across this info and thought it could help someone.
LEV Explorers/Mountaineers are equipped with California CARB compliant cats and the PCM has been tuned to compensate for it. If you put non-CARB compliant cats on it, it'll trigger the Check Engine light if you don't re-tune the PCM for 49 state emissions.