i have a 98 Dodge Dakota 2wd 2.5 4 cyl. the mil light keeps comming on i have a obd2 scanner on lap top. the code it keeps coming up with is po420. i have changed both o2 sensors, cat and manifold. hear while back when it first started i had a new ecu put in it and then it was recalled saying it had the wrong operating system on it. ever since then has always came up with this code. what could be causing this problem.

 

hmm... have the PCM checked it might still have a glitch.
and make sure you have no exaust leaks, is the new cat welded in? mine kept throwing a code and finally i found the the cat. was separated from the resonator.


(now it's on all the time but i don't have an actual exaust but that happens)

 

The code comes on when the PCM sees no change in the O2 sensor signal from before and after the cat. The recall you are talkin about is that the PCM program had no way to determine if the cat was bad or not. I would have to agree and say that it sounds like a PCM problem. Usually if it was anything else, there would be other codes along with the cat efficiency. Thats especially true if you have replaced the cat already and are still having the same problem, assuming of course that you don't have any exhaust leaks as they can cause the code also.

 

No exhuast leaks whats so ever its all welded up. my friend has a obdII scanner snapon with the latest updates. sometime he can get into the computer to delete the code but sometime the computer wont let him in at all. not even to get the littlest bit of info. im thinking its a pcm problem. i have an obdII scanner on my laptop and sometimes i have to turn the truck off and on several times to get it to read the pcm. doesn anyone else think its a pcm problem or program problem with the pcm or what?

 

Well as is the case with any CEL code, the PCM can always be at fault. I have replaced several for that same thing....I would have a vehicle with a CEL on and after all diagnosis steps didn't find the problem and the PCM fixed it. O2 sensor problems included. If you know for sure that eveything is ok...cat, exhaust, o2 sensors (which means the upstream sensors too...) then a PCM would be the next logical step.