[Jundle]

My Problem: My furnace is pumping out tepid cool air

Last night my house started blowing tepid cold air. I turned the heat down until it "clicked" off from the thermostat in the main room. I went down into my basement and opened up my furnace. I had not changed the filter in the last 3 years and it was caked with dust. I have never interacted with a furnace before in my life, and I don't know much about them. I took the filter out completely and then closed the furnace (thinking that I could pick up a new filter the next day [it was around midnight]). I went back upstairs and turned the heat back on and it started blowing hot air again.

After a while it started blowing cool air again. I tried to turn the heater off at the thermostat in the living room, but even though I had turned the heat down, and heard the signiture "click" that the heater was no longer going to pump out heat, the heater continued to pump air out, and wouldn't stop. I even set the thermostat controller to "Off" instead of "Heat" or "Cool" and it still would not stop.

I went back to the basement and looked again at the furnace and noticed the LED indicator was blinking "Normal: call for heat". I opened up the furnace, and it shut down immediately (there is a black "off" switch that gets released when I open up the furnace face plate). Nothing jumped out at me as being really wrong.

I closed the furnace back up and went up stairs and the heater was still off. I turned it back on again (this time just testing to see if the heater would turn off when I adjusted it to the "off" state), waited for air to blow out (which it did), then turned the heater back off. This time, the heater shut down like it normally should.

The next morning (today) I turned on the heater and it seemed like it was pumping out warm air again. After about 10 to 15 minutes, it started pumping out tepid cool air again. I went and turned off the heat at the thermostat in the living room and the heater continued to stay on. I went down into the basement and this time, the led light was solid "Continuous on: Internal Control Failure (Replace)". I now have the faceplates off so that the unit is no longer on. I have turned the gas knob to the off position. Also, it looks like there is a lot of dust caked onto the fan looking thing, but I assume that probably isn't the cause of my current problem (cool tepid air) as it most likely still spins seeing as how air is pumping up into the house when the unit is turned on.

Also, it's been EXTREMELY cold as of late, more so than usual. I don't know if that might have anything to do with the problem.

Could the solid red led be wrong and that I don't have to replace it?

Does anyone have any suggestions of what I should do next? I don't know much about furnaces. Also, I don't have a lot of money.

NEW NOTE: NOW THAT I HAVE COME BACK HOME and have turned the furnace back on again to test it, the red light is no longer solid. It is blinking with "expecting heat".