2008 Chevy Cobalt heater not working

First be sure you have the controls in the right positions,and they function..Check the owners manual for this. Be sure your anti-freeze level is correct. Was any cooling system repairs recently done? If so,you may have air trapped in the system. You will need to bleed it. With the car warm and heat on full blast,feel the hoses that run from the engine to the heater core.they should be hot.One hotter than the other.If one is hot,and the other almost cold,you may have a clogged heater core.

start with checking the coolant level make sure it’s in between minimum and maximum. Look for leaks overnight under the car. If everything is okay it could be a dirty heater core. Get the car in normal operating temperature. Then drive it In higher rpm ( select lower gear) around 4000 rpm see if you can get some heat. If you do get some heat I would check the quality of the coolant (need a tester for that) get a radiator flush and heater core flush. If you don’t get heat at higher rpm it could be a blown head gasket. You should notice a lost of power. If you just got the car it might be difficult to notice.

The things that would cause the heat to not to work would be low coolant, bad thermostat, bad heater core, and clog in your coolant lines.