2004 elantra heater control only operates at #4 setting

2004 Hyundai Elantra a real melt down (or just humor to say battery died as all do often)
most batteries never last 10 years, ever.(my 50y records me)

1. check all fuses first cab and engine bay.
2. check battery voltage running 13 to 15vdc good less is alternator is dead, (3 fuses bad makes new alternators fail too)
3. you never put the battery in backwards right?
4. you never jump started the car jumper cables backwards right?
5. scan the PCM yet, I bet it tells DTC errors, that you missed.
6. PCM means DLC ODB2 16pin port, all USA cars have 96 up.
7. what does melt down mean and doing what? IDK.

Either heater control valve stuck or blend door is stuck depends on if you have climate control or standard controls. Also make sure it full of coolant.

If the blower motor only works on high speed then the blower motor resistor has burnt out on the lower settings, and will only send power on the high setting because it is straight battery power.

Check for a blown fuse. If there is no blown fuse then I woudl replace the heater fan relay.

Check that the heater core valve is opening when you turn the heater control on the console. It is between the engine and the firewall.

start with a thermostat...

make sure the cruise control cable to the throttle isn't too loose, if it is ok, check your brake switch. if it is sending the signal the brake is on it will not allow cruise to work and will do same thing as described about the set light.

Had the same problem You need to go to dealer and order an O2 sensor with California emissions. The wire will be longer and fit into the prong

trying to do the same on my 01. ready to change the bulb but can't find it anywhere.
anyhow, here's how you do it:

remove the dash by taking out 2 screws behind the ashtray and 2 above the instrument cluster
begin prying off the dash from the ashtray up, when you reach the heater vents stop. next take a flat head screwdriver and pry off the dash from the left side with the door open. work left to right till you reach the heater vents and it'll come right off. you don't necessarily have to unplug any of the electric connectors if you don't want to, but you;ll have to reach around the dash while working. all you need to do now is remove the heater control unit via the 4 screws around it, then unplug the electric harnesses from the back of it. make sure you remember which goes where. the bulbs are the 3 white (or is it 2 white and 1 grey) circles on the back of the control unit. just turn them with a flathead screwdriver and turn it over and give it a little bump to get 'em to fall out (or grab 'em with a needle nose pliers if necessary). anyhow, that's how it's done, question is, where do ya find the bulbs?

To replace the control module, you'll need to remove the front dash facia panel to access the virtical console modules. Four (4) phillips head screws must be removed to pop out the this panel.

The facia has several 'clips' which pop loose when you pry it out.

As you can see, there are four (4) screws in the environment control module. Reassembly is in reverse order.