The catalytic converter on my 96 5.9 Ram 1500 has gotten a big hole rusted through the front, so it's exhaust time. Weighing my options, but probably leaning towards fabbing up an upgraded system front to back. Already has shorty headers on it, so I'm leaning towards making a mandrel bent y-pipe into a 3" single tailpipe system. The truck is OBD II; it has the downstream O2 sensor - but the previous owner gutted the cat because it was rattling. Upstream O2 switches well and averages about .5v, so no issue there. Funny thing is, downstream O2 averages about .8-1.0v, but it never tosses the MIL lite or a cat efficiency code. Similar voltage readings on my 99 Dak 5.2 tripped the MIL light often - after I changed the cat they were about .6 upstream/.1-.2 downstream, which is what I consider normal, and of course no MIL lite. I would think an OBD II ECM would catch the downstream volts being higher than the upstream and trip the light. Any thoughts? Probably not going to waste money plumbing a $100 cat into the system if the ECM doesn't recognize the downstream O2. At this point I will probably fab the system with no cat with the ability to easily swap I one in if necessary.

So, I'm sure this truck isn't the only one not having a functioning cat - anybody else notice "lazy" ECM monitoring of the downstream O2's?