Originally Posted by lost
as far as I know, the tj has 3 cats and the last cat doesnt have a sensor on it, and the check engine light wont come on if you take it out.
if this info is wrong, just tell me...
it depends on the year. Some 2000's have the 3 cat setup. Prior to that was just 1.
The 1 cat setup has 2 oxygen sensors to control emissions. The upstream o2 sensor tells the downstream o2 sensor what to see. If they don't see the proper numbers they help adjust your computer to be complaint with OBDII regulations. Does it always work like it is supposed to? No. Will it jack up your gas mileage if they aren't working properly? Absolutely!
There are people that try to run MIL eliminators to fool the o2 sensors into believing nothing is wrong. But it won't always work. The catalytic converters are there for a purpose and they don't hinder flow enough to warrant removal.