Replacing O2 sensors. Which is which?
 

I have decided that it may be a good idea to replace my O2 sensors. Poor mileage, hard to start in cold, runs different based on weather, hard to start when engine is hot, PO told me to do it. Tons of reasons. I just now realized that there is an upstream and downstream O2 sensor. I only knew about the one near the catalytic converter. Do they both have the same function? Should they be replaced at the same time?

This is 96 XJ with 287K

Thanks in advance.

The upstream o2 sensor that's before your converter is the one that affects performance, the downstream after the converter is only there to trip a code if the cat isn't working right.

On a 96, the one involved with air/fuel mixture adjustments (via the PCM) is the upstream sensor. If you only replace one, make it the upstream sensor. The downstream is used to measure catalyst efficiency (emissions standards).

Almost any symptom that can be caused by an o2 sensor, can also be caused by a bad MAP sensor (on the throttle body). O2 sensors are typically not involved with poor cold-starts. As they take 60-120 seconds to even be used by the computer after a cold start.

Do you have an OBD2 scanner? they are invaluable for seeing what your computer is doing, and how it is adjusting the air/fuel ratio, and what it thinks the O2 sensors are doing. They can be had for about $25 these days on amazon (I use the BAFX Bluetooth OBD2 scanner for Android for my 96).