02 alero need to change the oxygen sensor but don't know which one it takes upstream or downstream? any help would be greatly appreciated. motor=2.2 4 cyl. dual overhead. thanks

If you had the computer scanned it should have given you a code. Bank 1 Sensor 1 is the front while Bank 1 sensor 2 is the one behind he cat. Get the code and it will tell you which one you need to change out. Without the code you will need to change both and since the front is different from the rear then it will get quite expensive.

You should still do them both.

thank you guys for the information. one other question i took the car today and had it hooked up to a diagnostic machine at napa and it only threw the oxygen sensor code, the thing is the car is stalling quite frequently...any ideas to why?? thanks.

Change the O2 sensors first to see what happens but this sounds like classic Crank position sensor issues. The CPS will start to go bad and cause stalling but it will not throw a code. When it goes totally bad the car will not start.