Usually the fault lies with a failed "Blower Resistor". This part can be found at Napa Dealers. Auto Zone does not carry this item as I had this same problem just one week ago. The part is $15.99 and is very easily replaced! Located under you glove comapartment on the blower/ventilation duct unit. Two screws or hex bolts (5/16 head) are securing the resistor set. There is a diode attached to the top of this resistor bridge, so be careful to remove gently from the duct work. I say this if you want to test for an ohms value and continuity. When this resistor set fails usually the low and mid speeds are not funcitning. Then all that is remaining is the high speed. I had this exact symptom. The resistor bridge is a high failure item and will have to be replaced at some time anyhow. My truck is a 2003, and the part from Napa will require you to remove the small plastic tabs from the ends of the mounting base before installation. This will allow the news part to fit. So any Q's and my email firstname.lastname@example.org So go for it Dude!
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IT could be one of several things: possible the blower fan motor is bad; or the valve that lets the hot water in from the engine to the heater core is not opening; or the door that lets the air flow past the heater core and onward to your vents is not opening. All these are controlled by your heater switch, via relays and vacuum lines. If the blower motor works well when you put power to it directly, but not when it is hooked to the car, a connection in the switch or a relay may be bad. if you get little or no effort from the blower motor when you hook it directly to a good battery, it's the blower motor itself gone bad. If you get airflow but not heat, it's the device which diverts the hot water into the heater core. and if its the door to let air flow over the heater core, it could be a relay, a vacuum hose or the switch.
Common question on this site. On late model cars, the vents are controlled by blend door motors. Small electric motors open and close doors to direct air flow. Chances are you have a blend door that is stuck or a failed motor. The default position is the defrost to make sure the windshield is always clear. Someone will need to look behind the dash to see if all of the doors will move on demand.
If both heater hoses are getting hot then you know the coolant has to be flowing and the core should be hot.
What you will have to do is locate the blend air door servo.
The blend air door is a door (or flap) between the heater core and the A/C evaporator (even without a/c there is still a door to control the cabin temp)
Early vehicles with the slider controls used a cable but the later vehicles with a rotary knob or digital buttons use a servo to rotate the door mechanism.
The servos can go bad and the switches as well.
But from the sounds of it, your door is not moving to allow the heat through the vents.
It sounds like the heater motor may not be operating. When you switch the heater fan on can you hear the motor running? If not then it may have blown the heater fan fuse, check the fuse's and then try it again. If the fuse is ok then it could be a faulty switch or fan motor.
Check any vacuum hoses from engine all the way to heater/a/c housing under dash,for leaks. Then manually move or put a vacuum guage on every vac motor you can see and get to. Your working your way back to the switch. If you want to you could start at the switch but % wise and degree of difficult your better to back in. Now a schematic will make the process easier. Hard to work on a system you don't know .Electrical or Computer sometimes play into these, I have an older S10 so no computer.
your blower resistor bad or the blower motor control switch is bad. no air flow from top vent make sure vent open.because you can manual adjust them not to blow air on you.if all is good.you have leaking vacuum hose or vacuum actuator.or air door stuck.the AC and HEATER control switch.is bad.its not allowing vacuum to open air door.
working on 4 and 5 speed that the resistor that broken cold air you got to move the selector to see the motor that move the trap hot and cold sometime when the motor was not move to the hot side for a long time its jam the motor on the passager side under dash put to hot and gently tap the motor tanks pierre