Originally Posted by Pre4runner
Search for this... its been brought up a lot but the upstream O2 is needed for a fueal air... its kind of a backup but you should have it...
No, it is not a backup, it is needed for the feedback control in the ECU. Without it, the ecu just throws fuel in based on a small table. Depending on other modes, this ammount may be rich, or it may be lean.
People that have no clue about automobiles and modify them (the person/s that removed Hombre's O2 sensors for him) should be beaten soundly.