bad O2 sensor code

Last night my check engine light came on and i checked codes and it said that I had 12/21/55... i know what they all are..but my question is...in my 1996 JGCL I6...how do i know which O2 sensor is it? There are 2 right? one after the cat converter and one before correct? Ive seen the one towards the front of the car...

Also...how do i know which one is bad? replace both? Thanks again!

Code 21 could mean both upstream and downstream it is not very specific. You really need as scan tool to Diagnost the sensors. the upstream is located near the inlet of the Cat the down stream is located by the outlet end of the Cat.Check the wirring on both sensors they have been known to have shorts or breaks because of the heat.

thanks cam7, my check engine light came off today...not sure why, but regardless im going to change the sensors out. thanks for the reply,