First off I did search first but could find anything that exactly pertained to my situation. This is a question about my grandfather's truck a couple months ago his CEL came on I checked it, P0420 catalyst system efficiency below threshold (bank 1), first we changed the o2 sensor after the cat. Cleared the code and it came right back. The I noticed there was an exhaust leak on the weld of the muffler. His truck came new with a 1 in 2 out flowmaster. (it was a regency conversion) Replaced the muffler with a new one, (off brand not flowmaster, but the same thing 1 in 2 out). This was Monday night, I cleared the code and drove for about 8 miles. CEL stayed off. Was going to an auction last night, about a 35 mile drive one way. The CEL came on after about 25ish miles. Went to the auction and went home. I checked it today and still P0420. Any ideas where to go from here?

 

Sorry, I forgot what kind of truck it is. 2002 ram 1500 4x4 quad cab.

I'm used to posting for me with the sig filled out.

 

What year, what engine???

 

Its possible the cat is on its last legs.. Going through that with my old mans Hemi at the moment

 

Sorry we posted at the same time (post #2) but I still forgot engine.
2002 with a 5.9

 

The 2002 still have the plenum issue.. might want to address that.. cat is most likely plugged up

 

Recommended order of things to do for p0420 is first, check for exhaust leaks. Second, replace the downstream sensor (make sure it is the right part and doesnt say Bosch on it). If those two fail, you probably need a new CAT.

 

Dont look for exh leaks after the cat. Look for them before the cat. Also check the exh manifolds and bolts for leaks, looseness and cracks

 

Ok thanks. And when I read check downstream o2 sensor, I thought look for leaks downstream as well. Well I'll do what everyone said and try to fix the problem. He just wants to sell the truck, so I'm trying to find a solution.

 

If you think about it, the O2s can only monitor what is upstream of it, not downstream.