Rich condition cause p0140?

1996 Buick century 3.1 can rich condition cause p0140 post cat 02 code no activity? Running very rich elevated fuel trims. New 02.. 450 bias voltage on scan tool.

Engine computer checks for O2 sensor activity by counting the number of times the signal crosses from high to low and back through the 0.5v mid-point. If the signal is fixed high or low either caused by a sensor or wiring fault or true fully rich or lean conditions it will flag up a code in your case P0140. A related code is P0420 which is triggered by too much activity from the downstream O2 sensor.

The downstream O2 sensor on your car is fitted purely to test if the cat is working, it plays no part in controlling the air-fuel ratio. The air fuel ratio is controlled by upstream O2 sensor.
For the cat to work the engine must supply the cat with exhaust gases with the correct mixture of unburnt combustable gas and oxygen to fuel the catalyst.
So anything that supplies the incorrect air to air fuel to the engine will cause problems which be detected by the downstream sensor.

Like Andy is saying, I very much doubt your P0140 has anything to do with the rich condition. Two separate issues, IMO.

What are the fuel trims at idle and 2500, exactly? There's only a few ways these engines run rich - the most common is a leaking fuel pressure regulator.

If you've replaced the downstream O2 and still get the P0140, then it's time to do some wiring integrity checks on the downstream O2 signal circuit.

Does the pid for voltage on the scanner for that sensor ever change or does it show bias voltage all the time?

If the latter is true, then check wire integrity as allready recommended....

Thanks for all the replies. Newer vehicles post cat 02 play a part in fuel trims. Bias stays pretty much the same. Going check bias at sensor and ground circuit.