Thats because the downstream O2 sensors notice that the cats aren't working properly anymore. I believe there is an electronic solution to this but I can't remember if you need a chip to solve the problem or what. Maybe somebody else can chime in.
THANX FOR THE REPLY...WHAT KIND OF CHIP? OR DO I NEED A PROGRAMER?
Here you go. I knew there was an electronic fix to this but just couldn't remember. What you need are called MIL eliminators. They plug into the oxygen sensors and trick the computer into thinking there are cats installed.
yea i sure did i made a search to try to buy some and i found a website that has a diagram along w\ part #'s for radio shack ....so if anyone needs them i can provide the info....thanks again...p.s. it only cost about 20 bucks for all four