with a p0420 code how can i tell if it's the o2 sensor or the cat? Car started bogging down, not want to run right.

 

I would say the cat needs replacing. How many miles on it? I believe the 02 sensor will have its own code.

 

Basically, P0420 is only caused by an exhaust leak or a bad cat.

Check whether the cat is clogged.

 

replaced o2 sensor. cel gone, but car still boggs down and runs like crap. I listen to inj. to make sure they are working. I swapped the spark plug wires from my sisters lx and still same problem. cat is good, and fuel pressure is good. It has autolite platinum plugs in it, would these cause a problem?

 

How do you know the cat is good?

 

when i removed it i could see light thru it pretty good. plus it did the same thing with no cat.