still having trouble with engine code po420. i have changed the cat. manifold, muffler, both upstream and downstream o2 sensors. had the engine checked by my mechanic but we cant find what is causing the mil light to come on. the truck get 13 to 14 miles per gal and it has a 2.5L 4 cyl. in it what could be causing this problem.

 

I had a van in the deaership once that had a very similar problem. A rat or something had gotten under it and had chewed the o2 sensor harnesses up. Had a code for o2 sensor stays at center (forgot the P#) but after replacing both sensors, I had the same code. I went through all the wiring from the o2 sensors to the PCM and all was well. Turned out the rat shorted the PCM and thats why it was throwing a phantom code. Sounds like you have the same issue here. I would check all the wiring from the o2 sensors to the PCM and unless that turns up any problems, I think a PCM replacement is in order.