if you have a bad precat, it will also fool the computer into thinking the cat has been replaced.
the defoulers come in a pack of two. drill a 1/2" hole in one, so your o2 sensor will fit, screw that one into the other. those go in the exhaust and the o2 will go into it next. what it does is allow less exhaust get to the sensor. if the gases aren't within specs, the smaller amount of gas fools the computer into thinking everything is normal (example: your car is running lean. that means there is too much o2 in the exhaust. with less gas getting to the sensor, it will read that it is or near stoich). it does not work 100% of the time.
the help part number is 42002.
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