how many 02 sensors
 

want to change all my oxygen sensors time with my miles. anyone know exactly where they are i know of one in back of engine bay near plug wires easy for me to do is there 2 more or one more? and part or part numbers for them from gm parts direct thanks im having trouble finding these if its 3 total or 2 total?

there's only 2 , the 1st is in the engine bay near the coil packs and the 2nd is underneath the car after the cat.

One upstream, one downstream.

They are easy to change to answer the second part of that. The downstream will be the most difficult and you want to make sure your car is completely cool before you start trying to change that one. The downstream is probably rusted in there pretty good, so you may need a breaker bar or something similar to free it up.

you don't have to change the rear one, it doesn't affect the engine in any way.

then why do we have it

for it to set off the SES and display a code so you fail emissions? it monitors catalytic converter performance.

why the hell does autozone have 2 different 02 sensors for each stream? they are both direct fir thats wierd theres 2 universal and 2 direct for each stream how the hell do you know which to get?

The ones these cars came with are heated, the universal ones are not. But I don't remember the details on if the heated ones are required or not or why.

I have a universal heated O2. You want heated.

Why do they have 2 different sensors? Different brand? Different price.

check this out,
both bosch different numbers and different prices both named before cat then same 2 for after cat very strange its gotta be some kinda error or maybe its kinda like oil filters where u can use a pf52 or pf47