99 Regal Check Engine/AC problem

I had a sevice engine soon light on and the dealer found it to be a faulty o2 sensor. A couple of days later while on a road trip, the light came on again, I was away for a couple weeks and couldn't bring it to the dealer. The light went out, but now the AC only works on the passenger side, warm air only comes out on drivers side(wierd). Any input would be helpful in knowing if problem could be related or not. Thanks

Which 02 sensor was replaced the front or after the CAT convertor?

The dual climate system uses temperature actuators to control heat to the passenger and driver's side. Sounds like the passenger side one may not be working right. It is an electrical driven motor. Open the glove box and to the left/upper towards HVAC housing you will see it. While adjusting passenger temperature you can visually see if this is operating correctly. Watch the center shaft to see if it turns clockwise or counterclockwise.

Also, check your coolant level in the radiator and at the coolant reservoir separately and make sure both are full.