I got one of those o2 sim. for my car, for secondary oxygen sensor(cat) and when i check it there's only three wires. the o2 sensor on my car have four wires(obd2 two black one white, one green)

anyone know how to hook it up?

here some picture of that thing.

 

Its a 3 wire sim but if you look you have to put the 4th wire into the pigtail end of the thing you have.

It looks like a casper electronics sim, shoudlnt be hard to install. You need to find the 12v power supply. Thats your positive end, theres directions in your manual on how to do this. looking at the flat end of the stock O2 sensor, 2 of the wires are sensory terminals and the other are power wires. You should be able to do this is you have a multimeter.... i sold it before i decided to install it. SO.... sorry i cant be of any more help.