about two or three months ago i buried m truck so bad i got water up to the cat on my truck, both o2 sensors are bad, it destroyed the cat so i gutted it, i put a cherry bomb on, i still run both the origianal sensors, now i am getting ready to change one of them, the truck smokes, and runs very rich with horrible gas milage, what sensor will stop this? and i will also be doing a tune up on the truck, plugs wires air filter all that stuff, but i dont have money to get both

 

The sensor before the cat controls the fuel mixture.

 

ok, then what does the rear one do? and if i change that one sensor it shouldnt smoke and run very bad gas milage anymore? cause i put a comp to the diagnostic port and it says that all cylenders are running to much fuel

 

The rear one just measures the efficiency of the cat. It doesn't affect the mixture at all. You're going to get a code about cat being below efficiency (I don't remember the exact code but the scan tool should tell you the meaning when you pull codes). There are simulators available you can put in place of the downstream O2 sensor to get rid of that code. I've never used one myself but from what I've read they do work.