When to replace the o2 sensor

01 4.0 120K

Is the O2 sensor an item that is usually replaced at a set interval or is it an item that usually just works until it breaks.

Jeep runs fine just trying to clean up after some neglected MX and a long sit.

80,000 miles tends to be a long, healthy life for an O2 sensor.

I have over 300K on mine and it's still within specs, just had a new jasper engine put in last thursday BUT I plan on ordering both up and downstream sensors in the next week or so as long as I have done as much as I did last week on it. I just kind of ran out of cash after coughing up over 2 grand on the motor and $200 or so on the installation kit and new clutch. usually the first indication of a bad O2 or a going bad O2 is milage taking a hit of 2-4 mpg, then it needs to be checked.

I had a recall O2 sensor in one motor that worked just fine. Replaced it and the idle suffered. Had another that turned out to be fairly shot, the replacement wouldn't idle worth a darn. The replacement for that one, seems to work just fine.
The manufacturer and price don't seem to have much to do with the whole process. It seems if your ECU wants to work well with the info it's receiving from your O2 sensor, everything is fine until it isn't. And replacing the O2 sensor and having acceptable results, is sometimes just luck.
A couple of old sayings pop to mind. If it ain't broke don't fix it and new isn't necessarily better.

Ive always had good luck changing the 02 sensors at about 100k miles, that goes for all the makes Ive had. Just my opinion, but Id rather change it ahead of time as preventive maintenance, at a measly 50 bucks, than to have it go out and then try to determine what the heck's wrong. Ive always used the Bosch brand, and although I know some of you guys dont like those, every one has worked great for me. So I guess just go with what works for you.

Generally, O2's get "lazy" after 70,000 or so miles. This equates to bad gas mileage (running rich), and could result in SMOG test failure in states that have mandatory testing. Lazy means they respond slowly to the changes in sensed O2, resulting in an engine not switching between rich/lean as often as it should. The switching is necessary to the proper operation of the catalytic converter. Extreme outputs from the O2 usually set codes and turns on the check engine light( post Reneck(sp) systems). O2 sensors can be bench tested with a propane torch and and digital voltmeter. They also can be tested on the car using a digital voltmeter and propane to richen the exhaust in order to observe the O2 reaction, and time to react. A Google search will get you tons of info on O2 testing.

I just resensored my 1988 XJ at 170,000 miles... It took me about a month but i was able to get every sensor off of ebay for under 25 bucks a sensor. The engine rusn smoother, starts better has no hi idle at start up.

I got an NGK o2 sensor for 20 bucks
Standard Motor Parts Map for 10
Coolant temp sensor for 10
etc... etc..

i think its worth replacing the sensors as PM esp i you dont know if the mx on the car was kept up...