HVAC heater blend door actuator is not working

This is the most likely reason

https://www.fixya.com/cars/r29872074-vehicle_cooling_system_no_heat

It could also be seized blend door actuators not directing the hot air. Or the switch unit.

https://www.google.com/search?q=2003+Hyundai+Elantra+heater+does+not+work

...

where are they,, HVAC sure
1 blend is common (hot to cold blend)
well 3 or 4 actuators
depends on if HVAC has manual controls or climate control or with dual controls. (an option day 1 new)
the answer is here, the service manual
be sure to scan the PCM see HVAC errors (lots)

https://eautorepair.net/Marketing/Default.asp

it is a complex system for sure automatic version
post real problem first.
all the damper doors need to work right.
if a heat problem>??? there is one test to prove problem is
inside cab or in the engine bay
we measure water temperature,(coolant) at the 2 tubes passing through the firewall.
the 180f both lets you know the core is fully hot and the problem
is dampers, struck in cold mode blocking heater core air flow.
beyond this, scan the PCM see 30-+ HVAC errors
then next we look all all acuators, for sure blend to be sure al work
in the full range of movement, damper door axles.

A heater core is a component inside the HVAC box (located under the dash) that is heated by engine coolant. Vent temperature is increased by routing air through the heater core before it comes out of the vents. The temperature requested by the HVAC control module determines how much air passes through the heater core and how much air bypasses it. This event is called air "mixing" or "blending" which refers to the hot air and cold air ratio. Air routing through and/or bypassing the heater core is carried out by the use of a blend door and blend door actuator. Over time these two components can fail mechanically which often results in abnormal noises and improper positioning leading to undesired temperatures. Now, higher-end A/C systems with dual-zone climate control have multiple blend doors and actuators; this is how how the two different temperature "zones" are achieved. A failure with only one of the zone's blend door/actuator would result in desired temperature out of one zone's vents and abnormal temperature out of the other zone's vents. You can read more here: https://www.yourmechanic.com/article/what-is-the-difference-between-climate-control-and-air-conditioning

Air temperature is controlled by the blend door actuator, sometimes called the air mix actuator. This actuator moves a door inside the HVAC box that routes incoming air through the heater core to be heated, around the heater core to prevent being heated or anywhere in-between to achieve the proper "mix". It would be very odd to have only one extreme or the other while adjusting the temperature knob...this could potentially be a controller issue as you suggested. I think what might be more likely though is that the actuator has failed in the full-heat position resulting in cold air initially then hot air after the engine warms up (heating the heater core). In either case, the blend door actuator should first be checked for proper operation, then the inputs (circuits from the HVAC controller to the actuator) should be checked for proper signal. You can read more here: https://www.yourmechanic.com/article/how-to-repair-a-door-lock-actuator-by-marvin-sunderland

First, check coolant level. If it's low, it'll blow cool air. Second, check the fuse for the HVAC system. The mode door actuator and mode door controls WHERE the air comes out and the BLEND door actuator and blend door controls the air temperature. So do a search for the locations of those items on your car and check their operation.

NOTE: LHD model shown. RHD model similar. All vehicles are equipped with a common heater, ventilation and air conditioning (HVAC) housing assembly (1). The heating-A/C system combines A/C, heating, and ventilating capabilities in a single HVAC housing mounted within the passenger compartment behind the instrument panel. The HVAC housing assembly includes:

1. Mode-air doors (2)
2. A/C evaporator (3)
3. Recirculation door actuator (4)
4. Air inlet housing (5)
5. Recirculation-air door (6)
6. Blower motor (7)
7. Blower motor resistor (8)
8. Heater core (9)
9. Air distribution housing (10)
10. Blend-air door (11)
11. Blend door actuator (12
12. Floor distribution ducts (13)
13. Mode door actuator (14)

climate control is control by a controller or computer which read inside car temperature sensor and ambient sensor.best have vechicle check at garage you need diagnostic equipment to trouble shoot climate control.many things cause climate control problems. could have faulty temperature blend door actuator or bad sensor.

Check for correct coolant mixture. Also you could have a thermostat that is stuck completely open causing it to flow straight threw the engine block, cylinder heads, heater core, an radiator without stopping long enough to heat the coolant up to provide cab heat. It could also be a clogged heater core. Or a blend door on the HVAC box that is not allowing the heater core door to open. Hope this helps.

The most likely cause is a failed blend door actuator; however I recommend a little troubleshooting. First, can you hear a clicking sound behind the dash when you turn the setting to hot? If yes, it is certainly the actuator stripping its gears. If not, check to make sure the heater hoses are both hot as they connect to the heater core at the firewall (in the engine compartment). If one or both of the hoses do not get hot, there is a blockage caused either by an air bubble or a stuck water control valve in the heater hose. If the hoses are both hot, the indication is again the blend door actuator.

Below are instructions for replacing the actuator. Look at the diagrams carefully--the big arrows point to the front of the truck and the actuator is on the side of the heater box above the outlet duct that runs to the driver's side of the floor. Also, the actuator is expensive, so I recommend you make sure it's bad and shop around a bit to get a good price. To check out the actuator, first see if you can turn the blend door by hand. If you can't turn it, maybe it's jammed. Also, reconnect the actuator after removing it and see what it does when you turn the heater to hot.

Removal & Installation

1. Remove the floor air outlet duct extension (1) from the floor duct (2).
   

   

Fig. Remove the floor air outlet duct extension (1) from the floor duct (2)

Disconnect the electrical connection at the blend door actuator (#10 in pic below).

Fig. Blend door actuator and related components

Remove the blend door actuator mounting screws from the HVAC module.

Remove the blend door actuator as an assembly with the actuator cam.

Fig. Remove the blend door actuator mounting screws from the HVAC module

To install:

1. Install the blend door actuator as an assembly with the actuator cam.
   1. Line up the heater/defroster valve lever with the blend door actuator cam slot.
   2. Line up the A/C valve lever with the blend door actuator cam
   
   
2. Install the blend door actuator mounting screw to the HVAC module.
   1. Tighten the blend door actuator mounting screw to 14 inch lbs. (1.6 Nm).
   
   
3. Connect the electrical connection at the blend door actuator (10).
4. Install the floor air outlet duct extension (1) to the floor air duct (2).

Picture of actuator:

I would look at the air blend doors first, it sound like they are not working or your control in the dash is not working send a signal to them. they work by changing the air from hot to cold or in between.

Avalanche, Sierra And Silverado

1. Remove the floor air outlet duct extension (1) from the floor duct (2). Remove the floor air outlet duct extension (1) from the floor duct (2)
2. Disconnect the electrical connection at the blend door actuator (10). Blend door actuator and related components
3. Remove the blend door actuator mounting screws from the HVAC module.
4. Remove the blend door actuator as an assembly with the actuator cam. Remove the blend door actuator mounting screws from the HVAC module

To install:

1. Install the blend door actuator as an assembly with the actuator cam.
   1. Line up the heater/defroster valve lever with the blend door actuator cam slot.
   2. Line up the A/C valve lever with the blend door actuator cam
2. Install the blend door actuator mounting screw to the HVAC module.
   1. Tighten the blend door actuator mounting screw to 14 inch lbs. (1.6 Nm).
3. Connect the electrical connection at the blend door actuator (10). Blend door actuator and related components
4. Install the floor air outlet duct extension (1) to the floor air duct (2).

C/K Series

1. Disconnect the electrical connector to the mode actuator that is located on the left side of the air distribution case.
2. Remove the screws.
3. Remove the adjuster links from the control lever.
4. Remove the mode actuator.
5. Adjust the mode actuator by completing the following steps:
   1. Connect the electrical connector to the actuator.
   2. Turn the ignition switch to the RUN position. Allow the self-positioning of the actuator.
   3. Turn the ignition switch to the OFF position.
   4. Remove the actuator from the electrical connection. Blend door actuator-C/K Series

To install:

1. Install the mode actuator.
2. Install the screws.
3. Install the adjuster links to the control lever.
4. Connect the electrical connector.
5. Test the circuit operation