Heater not working

[campbmic]

I have a 2003 yukon xl with 127k miles. About two months ago the heater stopped working. No warm air at all coming out. I'm guessing it would be the heater core? I've never troubleshot a heater before, what do I need to do?

Thanks!

 

[OR VietVet]

If the a/c works but no heater at all then suspect a blend door actuator but need to address the basics. When you say no heat do you mean it blows cold or just blows luke warm? May have a thermostat problem. Do a search on here for this problem and I bet there is a lot of threads about this.

 

[mharders]

Need more info as posted above. However, these trucks are notorious for heater problems. I had a blend door actuator fail with my rear air. With the front climate control it would work intermittently (only blew on highest setting) then one day completely stopped working. My issue was caused by a bad blower motor resistor. Again need more details to try and help

 

[SLCHOE]

Well we need to narrow it down a bit.

Start up your truck and let it run or drive it around the block and get it up to normal operating temp.

Turn the heater on and pop open your hood.

On the passenger side of the firewall are your heater hoses. They should both be hot.
If one is hot and one is cold or lukewarm, you have a plugged up heater core. If both hoses are hot and about the same temp, you may have a blend door issue if the air blowing out of the vents is cold.

 

[campbmic]

It wasn't blowing any hot air at all. The air that was coming in was the same temp as outside. After trying it on a few different occasions I didn't try it again. This was a few months ago. I just drove it 400 miles and tried it again and it works now. Haha lets see how long it will last, I might have to come back in a few days