I have a 2006 chrysler 300 it will not reach operating temperature. It warms up while idleing like normal, but once I start driving on the highway the temperature goes back down. This happens when the outside temperature is below 45 degrees. I have had the thermostat replaced and I still have the same problem. This also causes my heater to blow cool air

SOURCE: Heater blows cool air & never heats up.

I changed the thermostat. It works like a charm now!!

SOURCE: I have a 2002 Chrysler Town and Country minimvan

May be a bad head gasket check oil for signs of coolant mixing in there may be a cracked head or even a bad head gasket. Going by what you said that there is NO visible leak the only other way for you to be loosing coolant if for it be burning out by engine or mixing in oil. To check for mixing check under oil cap for creamish-while residue or oil looking like milk shake. Good luck and thanks for using FIX YA

SOURCE: pull to a stop light and the heater starts blowing cool air

sounds like you need to either...add anti-freeze solution to your radiator...or worse case...coolant pump is starting to fail or heater coil is clogged..if coil is suspected to be clogged....do a radiator flush...reverse and foward flush the coil only...hope this helps

SOURCE: My 2004 Chrysler Sebring blows our cool air only

When the engine temp looks normal on the gauge but you get no heat inside unless your keeping the RPMs up by driving it,It usually means the fluid in the radiator is very low.wait until it is cooled completely and remove the radiator cap and fill completely,Check resovour and fill to correct level,

SOURCE: heater blows cold air ,at idle and blower motor

the heater control valve is not opening all the way.
look under the hood on one on the heater hoses .
you should find a vaccum controled heater hose valve.
make sure your not low on coolant also.
also make sure none of the heater hoses are pinched off.