egy amerikai fellow ugyanerre a problémára ezt a választ kapta:

Check to see that the blend doors are not hanging up in the heater duct. If okay, check the coolant level in the radiator, not just the overflow tank. Do this when the engine is cold...you don�t want to take the radiator cap off when the engine is hot. If your car is equipped with a heater control valve, make sure that it is operating properly.

ez nekem nem mond semmit,még ha értem is:)