An Exhaust Code is coming up

[Snagged]

I have an off road H-pipe, MAC, and a 3 inch MAC cat back system on my car. The rest of the exhaust is stock. I'm getting a code of P0420 for an exhaust sensor. Two of the O2 sensors have already been changed and the code is still coming up. The one on the driver's side pipe and one upstream. I'm wondering if I need a new MIL eliminator for the combo???? Anyone know if if that would fix the problem for this particular code?

There are no catalytic converters on the car.

 

[4601S/C]

That is a catalyst efficiency code. Which means your rear sensors are trying to read the exhaust stream. Normally the cat's would clean up hydrocarbons before they each the rear sensors. I would get eliminators, or turn off the rear sensors in your tune. The front sensors are the ones doing the important work.