P0420 cat problem
 

Okay so my check engine light comes on and off, it's mostly on. Bad cats, I need to get my truck smogged asap I recently bought it so I can't switched the title over or anything till I get it smogged. I found a place that would replace both for 500 flat. But they use aftermarket cats, is that bad? What other options are there? Seems I get worse mpg when my check engine light is on too.

You might want to figure out just what is wrong.

That code means there is not enough difference in readings between the front and back sensors.

Could be sensors , wire damage, exhaust leak or ECM the cats themselves.

It would be bad to change the cats and not fix the problem

Do your cats need to be C.A.R.B. Compliant ??

Same thing in my 4runner. Montana doesn't require a cat at all so I have just left it for now. I have a pickup that I took the cat off completely and straight piped it and it ran much better! Are there emission laws where you live? If there's not just cut it off or drill it out.

I'd guess something is wrong with the car vs something like a leak.