I can only give you a couple of ideas.
O2 sensors are capable of generating between 0.1 ( lean ) to 1.0 ( rich ) volts.
The pre-cat O2 just switches faster ( responds to mixture changes quicker ), than the post-cat sensor.
The post-cat sensors purpose is two-fold. It watches the pre-cat sensor & also " measures " the efficiency of your converter.
If you are running out of fueltrim, your # 1 sensor probably needs replacing.
Gee, I sound like an O2 sensor salesman, which I am not !