when I first start up my 2005 2500 HD and turn on the heater it will blow cold air until i drive it for a few miles. Even if I let the truck warm up for 5-10 minutes and the engine is at operating temp, there still is no hot air through the heater until I drive it for a while. Any thoughts??

I hate to ask the obvious but is the coolant level ok?

Oh and if it is a diesel with a stuck open thermostat it will do this also.

Heater core may be plugged.

When'* the last time a coolant flush was done?

If it was a bad thermostat you would not be getting heat from the heater, but only because it is preventing the engine from coming up to operating temperature. If the engine is coming up to operating temperature within a normal time, the problem is not the thermostat.

Has the coolant been changed recently? I would start by bleeding any air out of the cooling system. Is there a bleeder screw on top of the thermostat housing? If that doesn't work the next step would be to flush the cooling system. If you do that, you can remove the heater core inlet and outlet hoses and hook up a garden hose to the heater outlet. This will allow you to back flush the heater core and check for flow (or blockage).

Could be a bad outside air temp sensor or depending on what HVAC your truck is equipped with it could be a cabin air temp sensor. Anymore unless you have a tech II to talk to the HVAC unit it'* hard to diagnose problems like this.

Boy do I feel stupid!! It'* amazing what getting your coolant up to full does for your heater! Once the mornings get colder here i will be able to see if it is more serious than just low coolant. Thanks for the tip 75 racer

No problem, I hope you find the sorce of your coolant leak. If its a 6.0 gas motor I would look at the water pump gaskets.