Downstream O2 is after the cat and that monitors the cat'* efficency and exhaust emissions. You may only have one "upstream" o2. To tell you would have one in the front exhaust manifold as well as the rear one you already changed. Still the downstream sensor should have no bearing on engine operation. It mainly tells you the cat is possibly bad and emissions are high. That may be your problem. Your cat could be partialy clogged. I myself have to get one soon.
Also after you replaced these did you have the codes erased? If not you can disconnect battery for 15 minutes to clear the codes.
What make sensors did you get?
The only dumb question is the one you didn't ask!
99 Olds Silhouette EXT 195K
03 Bonneville SE 127K
14 Chevy Malibu 4K
Last edited by azu; 07-15-2008 at 08:37 PM.