Can for a long time, i have this P0420 code: Catalyst System Efficiency Below Threshold (Bank 1). Now heres where im just confused. Had this code for a long time. Ive replaced the catalytic converter and both O2 sensors. 1 month later, check eng light came back on saying the same thing. Then 3 months later after the eng light came on, had to replace the engine and transmission when my trans went out and just replace engine since had 268,000 miles on it. And replace O2 sensors yet again. But still have the P0420 code. So used this trick to modify the sensor with the spark plug non-foulers trick and still nada. So any ideas?

 

The catalytic converter you replaced. Did you install a high flow cat say 200 or 100 cell?
I suggest to add to the rear O2 sensor a "mini cat"
This device has the honeycomb of a real cat. They can be bought off eBay.
They are about $30 for the ones I speak of.
There is cheaper ones but do not have the honeycomb and will not work.i have gathered as much info as I can and put here for anyone interested

http://imgur.com/a/l3sDM2fHope this helps