Just replaced the front O2 sensor and it consistently comes back up with the "stuck rich" and runs open loop until the code is cleared, but after only a short time it comes right back. How can it be stuck rich? Bad sensor? It's a brand new Bosch.
Take the old and new sensor and make sure the length are the same. Maybe your new one is going too far inward, maybe put a thin washer on than pretention the line than screw it in. Make sure there are no hard kinks or severe twist that would disrupt or give false signals. Maybe your new sensor is faulty, they're very sensitive, might have been damaged.
After my DP rebuild, the rear sensor had signal issues, too lean, than too rich, and I didn't touch it for a week and it varied... undid the lines, made sure no kinks and secured the line much better than factory did - hasn't come up again and I have no O2 spacers/washers/90 degree bends/etc!!
I put a spacer on the rear 02 cause I had some cels also.. Put the spacer on and some cels went away and some new ones showed up... I was like what the heck, my downpipe was not bolted tight enough to my turbo and the gasket was junk... That was fixed and all cels went away..