Heater/ ac control unit sometimes sticks to one setting

The A/C compressor will cycle if the climate controls are set to send air to the defroster vents, or if the controls are set to an A/C position. Running when on defrost prevents fogging. What is most likely your issue is that the blend door in the heater box is not shifting over completely to the heater core, thus you are getting the colder A/C air rather than the heated air. You could also have a kinked heater hose or partially clogged heater core.

Best bet is get a factory service repair book, it will show you where each one goes

There are several things you can do. Your car has a diagnostic capability in the Control head of the A/C-Heater. This is a mixed benefit because after you trigger the diagnostic test, you would still need to know which part it is referring to and how to change it.

For this, go to the Web and type in the subject such as Your year and make of car, and climate control diagnostic sequence.

The other most common bad part is the blower motor resistor. Check the Web for photos. Some blower motors are under the hood, some are under the dash. The under hood models have wires running from the heater blower motor across the Firewall to a small protruding part. It usually has 1 electric connector and 2 mounting bolts. 90% of this part is hidden in the ductwork.

Sometimes you need to change this part, other times you can cheat and poke the part with a stick and it will resume working.

How long will it work? It is one of those goofy parts that is about as dependable used as it is new. This also means a new part can be just as unreliable. Carry a stick in your trunk to poke this part.

Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)

In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.

Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.

Melted relay

A serious symptom of a problem with the blower motor relay is a burned or melted relay. Relays are exposed to high current loads and can sometimes get hot when they develop problems. In serious cases, the relays may heat up to the point where the interior components and plastic housing of the relay will begin to melt and burn - sometimes even causing damage to the fuse box or panel as well.
Since the blower motor relay is essentially the switch that directly controls power to the blower motor, the entire AC system will be left unable to distribute its cooled or heated air if the relay fails. For this reason, if you suspect that your blower motor relay is having a problem, have the vehicle's AC system diagnosed by a professional technician from YourMechanic. They will be able to determine if the car will need a blower motor relay replacement, or a different repair, to bring full functionality back to your AC system.

first off make sure u didnt blow any fuses if the ac fuse is blown it wont allow these buttons to opperate and if not you need to make sure its plugged in all the way sometimes the plugs get loose and will not make contact good ...if these dont work u will need a new controller they run from about $119 to $200 from dealer ..

May be the heater/AC control unit in the dash.The heat stays on the floor because the door actuater moter is not switching direction of air flow and as far as the warm air,if you engine is up to operating temp,then the heater control valve is not opening fully.Both are conected to the heater/AC control unit.

AC blows where you have your control switch set,if it only comes out your defroster have the vacuum check at the blend air doors or if you have a faulty heater control unit.

THE BLEND DOOR THAT CONTROLS HOT TO COLD SWITCHING IS STICKING. THE CONTORL HEAD IS ELECTRIC AND MAY BE CAUSING THE PROBLEM. HEAT AND AC ARE 2 COMPLETELY DIFFERENT SYSTEMS EXCEPT FOR THE CONTROLLER UNDER YOUR RADIO. HEATER COIL IS REALLY A HEATER CORE WHICH IS A LITTLE RADIATOR INSIDE THE CAR BEHIND THE DASHBOARD.

Control panel going bad.