p0146 1999 jimmy

Getting DTC p0146 on my 1999 jimmy. 139K miles. Which O2 sensor is bank 1? Upstream or downstream? I would guess upstream, but I don't want to replace the wrong one.

regards..

This is the drivers bank, 3rd sensor down from the manifold, could be post cat on this truck. Code says no activity, check the wiring first.

thanks for the reply. I haven't crawled under it yet to even know how many sensors there are or where're they're located. Just assumed there were 2, one pre-cat and one post cat.