nagging O2 sensor issues
 

I have installed a full exhaust on my 02 S. there is another thread regarding that install. i have had a weird problem from the get go. the secondary O2 sensors have been fine. the precat sensors are the ones that have been giving me fits. i have installed new post cat sensors and had to re-install the original pre cat sensors. i am getting an "aging sensor code" which code be true, but it was doing fine prior to being reinstalled w/ the new exhaust. i have a new set of sensors, but wanted to ask if anyone knows the voltages i should be seeing on an obdII scanner that would indicate normal at idle and steady 2500rpms? i'd like to figure a way to verify that the sensors are indeed bad prior to swapping them with the new ones. thanks for your help everyone. looked online to try to find the values, but no luck.
brad