99 model 3.0 have three cats. there are two small ones within a foot for the exhaust manifold followed by a 02 sensor on each side, then a large cat atfer the y pipe followoed by another 02 sensor. my motor is flex fuel, but the engines computer needs the info from the o2 sensors and the cats doing their job to properly manage the running of the engine. no cats is a bad idea in my book not including the fact that it is illegal to run no cats on a cat equiped truck. just my .02. if the truck didnt have them from the factory, it doesnt need them, but if it does, they are put there for a reason!