i just got my 1997 gmc z71 out of the shop, i had a new motor put in it. after i got i started checking my gas milage it`s 13 truck is stock. i1m getting a code for the o2 sensors and i beleive it`s tripping othe sensors. i haven`t crawled under it yet but how many o2 sensors does it have? i have seen 2 1 up and 1 down stream or the cat but are there others. i can replace them myself but i just want to be sure when i go get new ones i get all of them. thanks

 

One on each side of the cat. The upstream are for fuel schedule.

 

thanks