climate control problem

I have a 92 maxima. When the automatic climate control is active (either Auto or Economy) and set for 27 degrees c, it just blows cold air (the car has been running for 30 minutes and it is -10c outside).

If I switch to manual, hot air is blown through the vents.

My mechanic suggests flushing the system...but it seems to me there is heat...its just that the auto climate control is out of whack (some sensor perhaps??).

Has anyone else ever had this problem? Suggestions??

thanks in advance