I just bought a 2003 Evolution 57k miles and i have an O2 sensor that is bad.... i went to auto parts store and they told me 2 different kinds... im not sure which one i need to get, if anyone can help me on how to tell which one please let me know, thanks in advance

 

Probably an OEM plug and play unit versus a universal unit.

The difference being the OEM unit will plug straight in really easy, the universal unit will need to use the plug from your old sensor and wire it to the new one.

I'd pick the OEM unit for the ease of replacement, but you might save a few dollars with the universal.

 

Front or rear o2 sensor? I have a rear one if you want it

 

they said something about upstream or downstream O2 sensor, so im not sure which one necessarily needs replacing

 

Well what code is your car throwing? You have two o2 sensors and we cant read minds here so you need to let us know what the code is and then we can assist you!