I know there is numerous threads like this, but none of them had this weird combination of things going on. Although i think i know the answer is the thermostat, i will describe anyway.

It is about 6 degrees out tonight and i was taking a drive to wal mart. Heat starts to come on, then turns cold, sort of switches back and forth from warm to cold. I get to wal mart, buy some coolant, fill the reservoir up and the same thing is happening. the weird thing is that the temp guage is all the way to the left, pinned past the "C" and doesnt budge. So obviously its either a thermostat, heater core, or...i dunno. Im going to check the 2 hoses in the firewall in the morning unless someone has a better idea.

 

You may have air in the cooling system.

Your thermostat is probably bad.

Your heater core may need to be flushed out.