I have a 2002 Jeep Grand Cherokee with dual temperature controls for the driver and passenger. I noticed a few times that the heat did not appear to be working, since it was summer I ignored it but, it was a real problem. I took it to a shop and they said it was the thermostat that was sticking and replaced it. I got it back and the first two days it worked but not at perhaps 1/2 capacity, then nothing. As I inspected more closely, I found that the heat only operates on the passenger side and is dead cold on the driver side. What is causing this problem?
This is a classic description of a failed Actuator on the flapper valve for hot and cold on drivers side. It is located deep in the dash and will require removal of the dash. You will need to have the actuator replaced. This will most likely need to be done at a dealer.