mY 2003 CHEV MALIBU I REPLACED THERMOSTATE MECHANIC SAID IT WAS STICKING THEN I STARTED TO GET A ADD WATER LIGHT THIS LIGHT COMES ON AND OFF AND THE COOLANT LEVEL ALWAYS SEEMS LOW I CAN NOT FIND ANY LEAKS BUT NOW THE CAR IS RUNNING HOT AND THE TEMP GAUGE IS NOT SHOWING IT AND NOW THE HEATER IS NOT BLOWING HOT AIR

 

I had a similar problem with my 03, I changed thermostat and coolant level was low for about two days. I searched the net and found out i had to bleed the system..I bled it Problem solved, topped off and good to go.You might have air in your cooling system.

 

+1... there is a bleeder valve under, but near the throttle body. I believe its a white valve.

 

Thats right i forgot to mention that bleeder valve, It is located in front of the engine right next to the waterpump...3.1 engine.