2008 Chevrolet Tahoe LTZ A/C works well. Heater blows cold; it blows cold no matter how high the temperature is set. What could it be?

 

What is the temp gauge showing?

 

Check the two heater hoses going into the firewall in the engine compartment after driving 15 minutes; if not hot, see above, after confirming proper level of coolant.

If that's OK, strongly suspect a bad actuator under the dash, these control the air flow, like over the heater core or not. I had to replace one on my 2005 Yukon.