I have searched threads, but needed to see if I can get any more help.
My 2004 Blazer 4X4's heater blows cool air, all fan speeds work.
I checked the HVAC fuses, they were good. With the engine hot, at operating temperature, and running I touched both heater core hoses and they both were hot.
Could this be my thermostat is stuck open?

 

blend door actuator or hvac control are common issues. if both heater hoses are hot you have flow through the core.

 

Hello, i had similar issue with my 97 blazer and i replaced the rubber lines off of the canister that is mounted on the hood on the drivers side of my blazer and wahhh-lahhhh heat. My air conditioning was working also so idk. just try that.

 

you are not alone!

My 2004 Blazer ZR-2 has no heat either, noticed it this terrible winter!
Change thermostat, no go. Rad flush at Mr Lube, had heat for a day.
This is still an issue, but i've gotten accustomed to driver with mitts and toque.. LOL

Owned my Blazer for 6 years, and replaced thermostat 2 times and NEVER had "good" heat, just so-so

 

Agree with justaguywithtools, Since both your heater hoses are hot, you know the coolant is circulating through the heatercore. Your fan is blowing, so that isn't a problem. The air the fan is blowing just isn't blowing across the heatercore.

For what ever reason the Blend/Mode Door is stuck in the cold position. Most common cause is a failed Blend Door Actuator. In rare cases i have found debree and or damage to the box that won't allow the door to move.

 

when you move the selector from hot to cold can your the blend door move?
if not...I suspect the blend door actuator.
If you can hear it work...I would suspect the heater core might be clogged...even though you can feel heat on both heater hoses I would remove them and try and flush out the heater core with garden hose and some compressed air may be used...but be careful too much pressure will break the heater core...ie don't use a pressure washer.

Good Luck

 

Reverse flush helped fix heat and when I checked for codes it had bad thermostat, I did change thermostat which is key. and sometimes they aren't good out of the box!


Reverse flush really helped me.

 

yeah ive seen a few instances where both hoses are hot and its a clogged heatercore. i think what happens is only like 80% of the tubes in the heater core clog allowing hot air to flow through a few tubes but no where near enough to heat the truck. usually if you adjust from hot to cold you can kind of hear/feel it working.

wouldnt hurt to reverse flush it. if you dont know which way reverse is, just do both ways a few times