My gas mileage in increasingly getting worse and I'm not throwing any codes. I looked at my entire exhaust system so I can replace my catalytic converter and o2 sensors. While looking I only have 1 o2 sensor. The only one have is the up stream sensor. I looked everywhere for the down stream sensor and couldn't find it. I know the sensor is suppose to be a few inches behind the catalytic converter. All the exhaust is original except for the muffler I replaced last year. I felt all around where the o2 sensor was suppose to be but there is no fitting to plug in a second sensor. I looked to see if there were any extra wires but didn't find any of those either. Could I have a special purpose XJ?

I have a stock 87 jeep (minus the lift) it only has 1 o2 as well, but im getting about 14c/20h mpg
i dont know about your year specifically, maybe find a diagram online as to "exact" location?

96 and newer should have an upstream O2 and a downstream O2.

Well today I found out I have mini cats near my headers and I found the other o2 sensors. I guess I have a cali model XJ. If I have a bad cat would it be the mini cats or the big one?

The down stream O2 readings will tell you if the cat is bad if you watch the data stream with a scan tool.

The big one is not a cat it just looks like one it is actually just the resonator.