NO HEAT! All I get is cold air.

2002 Yukon with a 4.8. The heater was working fine about a week ago..(when my girlfriend last rode with me). I installed a new aftermarket air intake today and after 20 mins of being outside I thought I would jump in and feel some heat. Notta. It is a little warm and more cold. Less warm and colder. I installed my factory intake back on thinking it may have been the new intake but same problem. ANYBODY? Thanks!

And the Yukon only has 25k on it.

when you changed intake most likely some gasket matterial got stuck in thermastat, change thermastat

If that would happen, the themo would open and close and that would cause it to throw out heat instead of cold. I dont know.

is it possible you could have put thermo in upside down, but also you could have crud in thermo holding it open, or you could have pluged up heater core, feel the heater hoses are they hot and still no heat? if so heater core could be pluged, what does your temp guage say inside the truck? watch it when you start it, should go up to around 200 then drop down quick when stat opens, if so its not that more likely heater cores pluged.
but if you've been under dash lately, install a radio or such? you could have nocked a hose loose, if you need to send email to my home i can talk you threw it better

Check your antifreeze levels. If you don't have enough antifreeze, the engine will get hot as hell, but none of the antifreeze will be reaching the heater core to heat the inside of the vehicle.

All levels are fine. There is hot water running though the hoses. I just dropped it off at the GMC place. It should be under warranty. I will post back on the findings just in case someone else has the same problem. The Tech at GMC said it might be something to do with a computer. I dunnno. Why cant it be something I could do myself. lol Thanks for your post.

What would installing an air intake have anything to do with gasket material getting stuck in the thermostat, or with the thermostat at all...unless removing/installing the air intake requires the removal of the thermostat housing or hoses attached to said housing.

Just got back from the GMC dealership, they fixed it under warranty. It was the actuator unbalanced. I dont really know what that means but my heat works, no bitching girlfriend. All is good. Thanks all.