P0154 ERROR- Which 02 sensor is bad?

[sbmoto]

2003 Z4 threw a PO154- my OBD scanner says its Upstream 02 sensor circuit no activity detected Bank 2 Sensor 1...My Chilton manual is horrible and basically says nothing...Im trying to figure out which sensor is bad...before the cat or after and which one...the manual doesn't show a exhaust breakdown and I think I need to pull the entire exhaust to get at the front 2 02 sensors....the rears supposedly are easy ....

[jbh1989]

Bank 2 Upstream is pre-cat for the rear bank of cylinders. With the right tools you get them straight down from the top.

[sbmoto]

Do I replace both at once ? What do I need to get at them ? I'd rather not pull the exhaust manifolds...