My 2005 malibu variable heater/cooling knob will not work right. no heat will come out when I move the knob to the "heat" side of things

Check the actuator motor on the heater it controls the doors that direct the heat and switches to direct the ac air as well. It is located inside the car in the center or end of the heater box.

Could be numerous things. First off check the heat control valve to make sure it's working properly. If the valve is working properly then proceed to checking the heater core. See if your getting good coolant flow. If you are then go to checking switches and resistors. Most probable cause is heater core blockage or stuck heater control valve.

There is a cable that runs from the temperature control panel down to the duct work under the dash on the passenger side. This cable moves a door inside the duct work. The cable has came loose or the door is not moving. This door controls the temperature. So check that out!

I don't think its a variable resistor or rheostat. I think its an actual I/O switch that controls smaller solenoids maybe up to three that divert air in places and open a door or close a door to the heater core that should be very hot. It may be a malfunctioning switch or a divert er solenoid or one of the solenoid could be broken because it is just plastic. The last thing i would check (not to be rude) is your coolant level a low level wont circulate your heater core making it quite cool.good luck hope this helps.

You need to repair the heater core with a soldering gun.

If you do not have heat, or only have heat, IMHO it is most likey the AC/heater control unit. This is the part which has the AC/heat control buttons and knobs.

Easy Diagnosis:
The easiest way to verify this is to turn on the radio, press and hold the power on/off button and press and hold the scan-up button for a second (full second). The word "A/C TEMP" should appear on the left of the display and a number from 0 to 16 on the right. I suspect yours will have the number 0 (or 16 if heating only problem) displayed no matter where you turn the temperature control knob. If this is the case you could turn the temp. knob to full heat and see if it goes to 16 after pushing down or to the right on the knob.
In a working system, the number moves evenly in increments of 1, from 0 to 16 as you turn the temperature knob.

Easy Fix:
I was able to take mine out and solder 3 loose connections to fix it.

The beginning steps are the same as for any radio removal procedure and check for loose connections.

I'm glad I could help. I'm reposting the solution below in case anyone needs it in the future. Please be sure to rate it for me!

Pull the temperature knob off and verify the thin 12mm nut is installed and tight behind it. I have seen several of these loosen up allowing the knob to turn but intermittently have no effect on temperature.

Try spinning the knob real fast in either direction. Sounds stupid but it worked for me.

Possibility your nob/resistor in the nob is bad. The resistor changes current in corolation with nob position.

it could be the fan motor resistor, i would check the main fuse for the fan on the heater. if the fuse is ok then the resister is either gone or has become unplugged.

The location of the blend motor on my 2005 malibu is on the driver side, near the center consol, not easy to get to. Before you try to remove the blend motor, unplug the wires it, turn the temp knob to cool side, disconnect the pos side of the battery for a couple of minutes. Reconnect the battery, plug in the bend motor start the car and turn the temp control to heat side. If this does not work remove the motor and repeat. the motor my be jamed. I had the same problem, this might be a cheaper way to fix this instead of the dealer. PS if you remove the blend motor, be carfull to look at the orientation of the spring on the shaft of the door in case it pops off.