I was reading up on a bunch of ram stuff and most everybody who talks about o2 sensors talks about 2, one infront and one behind the cat. Mine only has one and they previous owner did a s**ty "custom" muffler job. I have a suspicion that my truck is suppose to have two. No CEL or anything, but who knows.

Its a 1999 dodge ram 1500 5.9 sport

No CEL, No problem!

Which one is missing? The front one controls fuel ratio.

Upwind controls a/f ratio. Downwind monitors catalytic converter efficiency. either one of them will through a code when unplugged and turn on the MIL. If the upwind is unplugged it can cause other problems... Id be checking to make sure the bulb for the MIL wasn't removed too.

If it was just the DS O2 removed, maybe an O2 simulator was wired in to get rid of the CEL.

So these trucks do have two then...I need to check all the wiring I suppose. I CEL works, so I guess its down to finding the original wiring for it and remounting sometime maybe? I am almost positive the O2 is down stream of the cat. Means my air fuel might be messed up? Maybe that and possibly a plenum is why I only get 8 mph, or that its just a gas hog to begin with...

The US O2 is in the front of the Cat between the two inlet pipes... the DS O2 is in the pipe between the Cat and the muffler.

The Downwind O2(after the Cat) is for the cat, It just makes sure its working properly. The upwind is before the cat and that is the one that control A/F ratio. The plenum leaking can cause the upwind o2 to foul and plug the cat. they both can contribute to your bad fuel economy.

Well, I don't have a US O2. I checked and its not there. I need to find out where the wiring for it went and check my intake mani for oil. There is some oil residue on the TB and airfilter housing, but not an excessive amount.

Connector should be up on top of the trans somewhere, close to the motor. Pain in the keester.....