How do I know what O2 sensor is the upstream and which is the downstream

One more question please, is there a difference in O2 sensors between the upstream and downstream?? Can they be swapped out?? Are they all the same?? I bought some online and they told me there is no difference.

So their the ones between the engine and the Cat converter ?

Yes, those are the up-stream sensors --- So then the down-stream sensors are between the converter and the tail pipe ...... Think of the catalytic converter as just a dividing line .. The up-stream sensor is ALWAYS in front of the converter and the down-stream sensor is ALWAYS behind the converter!