Ive heard theres 2 02 sensors but i can only find one on my 93 f250 (2wd 351 windsor) behind the cat, is there another 02 sensor closer to the manifold or just that one? thanks

 

There should only be one before the converter on your truck. Usually located in a small crossover tube between the two down pipes. Never seen or heard of anyone having an O2 sensor behind the converter on this vintage truck.

 

There is just one sensor for the stock setup. You may have heard of two sensors when people take this truck and convert the computer from Speed Density to MASS Air (needs two O2 sensors).