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?

Are you sure no one had the PCM electronics modded for the gutted converter. My sons SRT-4 Neon also has a gutted converter but the previous owner sent the PCM out to have the electronics modded so it wont set the Eng lite on. The downstream 02 sensor does more then most realize.

First it should switch much slower then the upstream if the cat converter is doing its job. If the downstream 02 switching matches the upstream then of course it shows the converter not working. Also if the downstream shows the car running to lean or rich by the downstream voltage then it can cause the PCM to change the switching point of the upstream 02. Basically if it see's its readings showing the car running a bit rich then the PCM can lower the switching point voltage to lets say .4 volts instead of the normal .5 volts. In doing that when the car is being driven lean by the PCM when it see's a voltage over .5 instead of it switching to drive it rich when it goes below .5 it will keep driving it lean until it goes below .4 now to help makeup for the car running on the rich side. And vise versa if the car runs to lean going by the downstream 02 voltage.

That's just two of the functions it has as the PCM uses the downstream 02 reading in some of its other functions which is to much to get into now. My first guess is someone had the PCM modified to not set the eng lite by the downstream 02 readings ? Ron

I am the 3rd owner, but I have access to it's history since day one. I had already asked the 1st owner about any PCM re-flashes, etc. Truck has the factory PCM. It has never had a tuner installed or the PCM re-flashed. Original owner was an auto shop teacher and kept pretty thorough records, and still maintained the truck when the second owner had it - he's sure the truck never had any PCM mods. Friend has a 97 Ram with a 5.9 that has an open pipe from the y-pipe to the bumper - no cat or muffler - the downstream is still in the intermediate pipe - and again - no code. I have not put a scanner on it to see what it is reading.

I know the basics of what the O2's read and their function, which is the reason I found it odd that the readings I was seeing wasn't tossing a code, and when I saw the friends truck with the cat missing, I started to wonder if the factory tune simply doesn't monitor the downstream O2's input very much, if at all. That's the reason for this post, to see if others have seen similar results without the converter working.

Iirc the switch to OBDII was for the 97 year though some vehicles started changing earlier. Meaning the second sensor on a 96 truck may not be truly functional.

These trucks went full OBD-2 for the 1996 model year. Also at the same time they went from the hydraulic transmission to the electronic one.

And FWIT in the Walker exhaust catalog - there is a downstream O2 bung in the replacement pipe for the 96 Ram 1500's, and that bung isn't there in the 95 pipe.


So while I know that the truck is set up for OBD II functions, I'm just trying to find out if they are actually using the 2nd O2 sensor for tuning, since I know the cat is gutted and that the readings should toss a code and MIL lite. And I'm 99% sure the downstream O2 hasn't been turned off in the PCM. I don't know who would even be able to do that on a 96 - dealer? Or would a hand held tuner have that option.

What happens if you drive with the downstream o2 unplugged? You will have a solid check engine light on but tuning-wise I would think you would be ok.

Maybe you have a good point here as I have heard a few people who said they gutted the cat and did not set a code on OBDII cars and I figured they had the PCM software modded ? I never heard anything about the early OBDII cars downstream 02 watching less but who knows. I know when working at the dealer in one of my training courses I must have got a 4 page piece of paperwork on the 02 system with OBDII and how it uses a formula called something like "Weighted Extended" or something like that as I actually forgot the full name but I thought to myself........how in the heck am I going to remember all this in the 02 formula ?? Well I just try to remember the basics of it and then remember where the paperwork is when I need to know more. It sure made the 02 system in the OBDII system sound very complicated with all it did using the formula it told about.

But anyway I did not mean to imply you did not know alot about the system as you may know more then I do about it and I did not mean to offend you in any way or what you know. But it is interesting to know why its not setting a code if no one ever messed with it ? Maybe they went a little easy the first year as I have seen things like that before when engineering wants to be sure system works good in the field ? Ron

Certainly no offense taken here. I asked for opinions or info and you gave a polite and well thought out answer. I appreciate the reply. From responses here and conversations elsewhere, I think they monitor downstream O2's but do not react to them. I will un-plug it and see if it tosses a function or heater code. When I build the system I plan on - (if no MIL lite) no converter, installing a new upstream O2 sensor, and an old O2 sensor in a bung downstream. Worst case scenario I'll have to cut out a section of straight pipe and toss in a converter, as I am not a fan of driving with MIL/dash warning lights on. Wish me luck.

I haven’t had converters on my trucks for years and never any codes or lights on the dash. All obd 2 trucks.