What exactly is the difference between the two? I noticed the factory ones have green plugs up front and blue plugs in back. Otherwise they look pretty much the same.

Can you plug one into the other color socket, will it work if you do so?

 

Upstream is the one that is before the catalytic converters (it controls your air/fuel ratios) if your upstream is bad you will usually notice the car driving badly.

Downstream is after the cats

 

they do different jobs. the upstream is used by the PCM for air/fuel management; the downstream one is there to make sure the catalyst is working. If the signals from the two sensors are too similar, you'll get a check engine light and code(s) for "catalyst efficiency."

if the connectors are different colors, they're probably keyed differently to keep you from doing that.

 

I guess I meant, do they function differently. I noticed a lot of the aftermarket replacement ones just have black plugs on the O2 itself, no color coding.