96 Ram O2 Sensor Function

[EvilSpirit]

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. Something I noticed is - truck is OBD II - 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 downstreams?

 

[EvilSpirit]

Consensus on other forums is that the PCM doesn't react to improper downstream readings with a MIL lite or code. Many report OBDII trucks with the cats removed not throwing codes. We shall see. Going to fab my exhaust without a cat in a way that one can easily be added if necessary.

 

[dapepper9]

Whether it throws a code or not is a bit of a crapshoot. Easy way to avoid it is just tie it to the frame rail where the cat mounted before. Easiest cheapest way to deal with it. That's how i have mine done as well

 

[EvilSpirit]

Shimming the tip out of the sensor out of the exhaust flow can desensitize them, too.