02 sensor question

Hey, guys, when I pull codes, I am getting: Code 91- "02 sensor not switching Bank 1 circuit fault (SS1). This looks to me like I should replace the 02 sensor on the driver's side (at least).
My questions: 1.Should I replace both at the same time? 2. do i need to use OEM sensor from the dealer or can I save some $ & use aftermarket found online (ex:oxygensensor.com). 3. Would the bad 02 sensor cause fouling of my EGR valve which has been recently replaced but is now doing what the old one did, causing a bit of a miss & occasional stalling? Thanks for any into...Mike

I replaced both O2's when 1 of mine went bad. You do have the "eliminators on the front 2 right?
You only need 2 working ones and 2 with Eliminators.
TK

Uhhh.."Eliminators"?? I just have the two sensors between the headers & the cat..This engine has been recently rebuilt & is in a different car. GT40 shorty headers, crosspipe into one large universal cat.

Beware of cheap O2's they will prob go bad in little time. if the O2 is not switching like you said and its stuck on the lean side the comp. will keep trying to richen the engine up so it could foul plugs and such

So it sounds to me like it WOULD foul up the EGR valve, too..Thanks for the info..Did buy a set of Borg Warner ones today & will install them tomorrow. It had Bosch in it before..Don't know if that's OEM or not.

i have seen alot of bosch o2's bad right out of the box