I have a 97 Cobra. 33k miles. It has an offroad catless BBK H-pipe and MIL eliminators. My check engine light came on with a p0420 code (bank 1 passenger side catalyst efficiency low) so I switched my the sides of my O2 sensors (left to right and right to left). Both the front and rear. The cel came up again but with a p0430 code (bank 2 driver side) after the switch. I figured I had a bad sensor so I bought new ones for the driver side (front and rear). The P0430 code appeared again with the CEL, so I switched sides of only the mil eliminators and no change - still the p0430. By the way the MILS are from American muscle. None of this is adding up, I hope someone can help!
Thanks
P.s. It seems like more exhaust vapors are coming from the drivers side tailpipe rather than the passenger side. I can also smell the exhaust fumes a lot. It might be running rich but I am not getting bad gas mileage (roughly 18mpg). When I pulled the O2 sensors out they were black. Not sure if that has anything to do with my problem. Not even sure if I am running rich or not.

 

Your mil eliminators probably went bad. There is really no need for them. Pull the rear o2s and tie them up under the car then plug the holes. o2's take in the outside air and the oxygen in the exhaust and create voltage. around 450mv is what the rears need to read to not throw a code. With it reading the same air on both sides of the sensor it will put out 450mv.

 

that will result in a check engine light as well because the pcm's monitor strategy will try to drive a change in the donwstream o2 sensors and then look for that change to occour, since it wont, the check engine light will come on.

 

Funny, my check engine light must be broke and the pcm totally forgot to throw the code in the last 5 months since my downstream o2s have been tide up to transmission mount.

 

The mil eliminators are working fine. I believe I have a problem with running rich and it's just throwing a P0430 code

 

Not with those codes. If you were running rich it would be p0172 or p0175. The code you got means your catalytic converters are not working properly. So if you have mil eliminators then they are not working.

 

As I mentioned, I did a test to see if the problem switched sides when I switched sides of the mil eliminators. It did not. That's exactly why I am confused. So let's say that the mils do work, what could be my issue? If I am running rich, would it affect anything?

 

No, if your running rich your upstream o2's would would read over 1v and the pcm would trigger p0172 or p0175 codes.

 

in order for an o2 sensor to generate ANY voltage it needs to reach a temperature of 600 deg f. the built in heater alone is not enough to do this, and your under car temp will not get close to this unless its on fire. he is looking for a solution not more bad info and made up crap like u have been called out for in the past

 

For anyone having this problem with P0420 and or P0430 codes...try spark plug defoulers. I went through 2 sets of American Muscle MIL eliminators as well as several new o2 sensors. Then tried the double stacking method with defoulers and haven't had a cel for the past 2 months