where are the O2 sensors located on a 1999 ford explorer?

[Quinton G]

My 99 ford explorer has idling problems. Idles too high at start and then idles too low. I also smelled gas fumes coming from the exhaust. I had it checked and the results of the problem are many. I believe its my o2 sensors. Problem is i dont know where they are located and how many.

 

[GAZR]

Some have two sensors.
Ones guaranteed in the catalyst. (Exhaust).

 

[Mazda man]

Exhaust pipe. One before the converter and the other one after it.

Good luck.