P0420 (M) 1/1 Catalytic Converter Efficiency Catalyst 1/1 efficiency below required level.

ram 2001 5.9

This code showed up yesterday, I just had the cat replaced (high flow magnaflo).
Could this be one of tha 02 sensor's? If one of the sensors were bad wouldn't it show a different code. Could it be the computer not seeing the OEM cat? Or possibly the something downstream like the muffler causing the problem.

 

this is coming from your rear 02 sensor, the one after the cat.
its not recognizing a drop in emissions when comparing the readings before vs after the cat.
replacing rear o2 might or might not eliminate the CEL.

 

Thanks dhvaughan, I will look into that. Last time i went to the local parts stores, advance, oreily, auto zone, I kept getting different answers on how many and locations of the 02 sensors. They were telling me things like is it the right or left or you only have one and such, they drove me nuts and didn't know who to beleive. So I put it on the back burner. I geuss I'll give it another shot LOL

 

california emissions have 4 o2's due to the dual cats, all others have only 2.
its possible that your rear o2 just decided to break during the trauma of the exhaust work. if i lived inside a hot exhaust pipe for 8 years i'd be cooked and broken too...