Have a 2003 Monte Carlo SS 3.8.
When sliding the drivers side heat, Ac blend bar to the AC section, works great, but when sliding the passenger side bar to AC it blows hot air?

 

The blend door for the passenger side is most likely the culprit, you can attempt to tap the blend door motor with a screwdriver, sometimes it will get a little more life out of it. You'll end up replacing it eventually. Something I've always done in my car, is adjust the temperature one side at a time, no radio on, no blower, you should be able to hear it adjusting if it isn't then you know thats your problem. I can always hear mine switching when I go from hot to cold or vice a versa. Little troubleshooting tip without having to remove anything, I think its common for those to go out though.

 

ndaguanno x2

 

Thanks! nadaguanno.
I will give that a try.