Yes I have over 170,000 miles on my stock O2 sensors. I been meaning to replace them but where can I find them.
1991 Ford Explorer
Thank you for your help.

They are located in the exhaust pipe before the catalytic converters. Some early models only had 1, but my '93 had 2 (one for left/right). The sensors will have a few wires comming out of them and attaching to the wiring harness at the back of the engine (access through wheel well to disconnect them).

If your not having any problems with them don't replace them. When they go bad they will throw a code.

Thanx for the info guys, the reason i'm changing them is because I have been getting bad gas mileage lately and people have said to try changing the o2 sensors.

I would change them...They tend to get "lazy" as they age. This results in poor gas mileage.. The sensor (if you have it) on the passenger side will require a special o2 socket (autozone rental). You only need the socket because of the cramped space. The driver's side comers off with a few adjustable wrench.

http://www.hpphoto.com/servlet/LinkPhoto?GUID=1ea81818-169b-2299-4340-366af0a270f7&size=sm

Your '91 should have only one O2 sensor. My '92 only has one. Just upstream from the catalytic converter where the Y pipe comes togehter. I think '93 was the first year for two.
One little trick, I find it easiest to get to the connector through the wheel well (just like getting to the #3 spark plug).

Hey thanks for the help guys, I plan to change them or it this weekend, i'm sure I only got one for I got the explorer the first month out in the market.