Hello new to the fourm and new to Ram trucks, I bought my 03 2500 HD with a hemi back in November, so far I love the truck, but I got the check engine light and it came up with a P0420 code ,so far I have changed the post O2, it also seems like I am starting to lose power and mpgs, I really think its the cat but before dropping $250+ on replacing it. I would like to get fellow owners opinion

 

Assume you reset the code after changing the downstream O2 sensor? You did the right thing swapping out the sensor first. It could be the CAT but it could also be an exhaust leak or a faulty gas cap too. The biggest giveaway that it may be the CAT would be a significant drop in mileage, but if the truck is new to you, you may not have anything to base that off of.

If you didn't reset the code after replacing the sensor, do that. If there is still an issue, it will come back but might take a while. If you did reset the code, did it come right back immediately?