Do I need to replace an oxygen sensor?

[dagod16]

How many do I have on my 2001 xj cherokee?
Do I need to replace i5 or wait till its bad?
Thanks

[4.3LXJ]

You have four. Up to you, but they do wear out over time and affect the fuel air ratio which can affect engine temp or milage. Best to replace all four, but the two upper minimum if it has been a long time

[dagod16]

I have 188k and havent replaced any. Hahaha
Which one first?
How much are they?

[4.3LXJ]

They aren't cheap, but the two uppers first. They are the ones that determine fuel/air ratios. The other two monitor how well your cat works

[dagod16]

But my mpg is right at 20

[4.3LXJ]

Well, OK. Just saying you need to replace them periodically. If your milage is good leave it alone I guess

[dagod16]

The problem is.... I now have a chance to tinker on the jeep. So which one would you recommend I should replace first? I looked at auto zone and they are only 40 or 50 bucks, not that bad.

[4.3LXJ]

The upper ones closest to the engine