2005 LS P0135 code

[HeavyDuty]

I've searched for this code. I know it's the O2 sensor, I just don't know which one. I've looked online and it seems to be only one type? Which location is it? The one by the dipstick or the one lower down?

I know I want to get an Ac Delco brand but not from the dealer. Any website suggestions?

Any help is much appreciated!

Thank you!

P0135 is bank1 sensor 1 thats precat...I just use NGK sensors thier website will give you the part number