I hoping someone here has an answer to this question... Does anyone know why some manufactures, of o2 sensors, have the same part number for Upstream & Downstream sensors while others have two different part numbers? I'm having an intermittent code pop up referencing the o2 sensors, so I've decided to just replace them all. I had one replaced immediately after both cats were replaced. Very possible the slow o2 caused the failure of the cat...so I'm done playing this game and want to replace them all. While checking online at AZ, RockAuto and Kragen, I've noticed a significant price difference between the parts (as to be expected). I just can't understand while they list the same part/part# with a description of "upstream" then list it again as "downstream", but some parts show both up and down in it's description... Thanks!