Grand Prix GT heater problem

I have a Grand Prix GT coupe 1999 and a few days ago the heater air on the driver's side started to blow cold - even when the passenger side air is blowing hot (as when I set the temperature to say 75F). I've tried all fan settings & playing with the 'dual' knob, but I cannot get hot or even warm air on the driver's side. Yet the passenger side air is blowing hot air.
Has any one seen this before and can you suggest what is happening & how to fix it?
Thanks

my guess is you have the 3.8 motor?
also, do you have the manual, dual manual, or dual digital climate control. from the sound of it, you have the digital, but just checking.

Check your coolant level. When coolant gets low, the drivers side will lose heat first, as it is fed from the top of the heater core (on dual climate control models. If the coolant level is ok, then it's most likely a temp control actuator. IF the coolant level is down, you may have an upper plenum failure that is causing coolant to get sucked into the engine. This is a quite common problem on these engines.

Thanks guys for the replies!
I'll check out the coolant levels when I return from traveling this week end. My mpg is still good so I doubt/hope the problem is not coolant migration to expensive areas. My mpg is 30+ if I drive like an old man (coasting & very slow acceleration) and ~23 if I drive for enjoyment.
I have the dual digital climate control & have played with that to no avail.
Thanks again.

Thanks guys for the replies!
I'll check out the coolant levels when I return from traveling this week end. My mpg is still good so I doubt/hope the problem is not coolant migration to expensive areas. My mpg is 30+ if I drive like an old man (coasting & very slow acceleration) and ~23 if I drive for enjoyment.
I have the dual digital climate control & have played with that to no avail.
Thanks again.

When you get a chance checkout the vacuum lines under the evaporator/blower assembly. Make sure all are connected and tight. On dual climate control the head unit (climate control) sends an electrical signal to a vent actuator. The one for the driver's side is located on the left hand side of the center of the dash. When you set driver's side temperature this actuator will move. It is a motor and will control heater air flow. If it doesn't move then you have a problem with that actuator, the head unit or wiring going to it. Of course, the passenger side has its own and operates independently. There is a diagnostics mode for the digital dual climate control system and you could try running it. It may or may not reveal the problem however unless an actual DTC is set. Good luck!

See link below:

http://www.midwest3800.com/forum/kb.php?mode=article&k=164&sid=e1009d4d6c16cc7ab926e15c95f19e39