I drive a 2003 Elantra, and my heater isn't working properly. When I am giving the car gas, it starts to warm up- and when I come to a stoplight, slow down or break at all, it turns cold again. I have had this problem before and replaced the thermostat. At that time, my car was also overheating. That fixed both problems until now, and this winter the heat isn't working once again.

Any suggestions as to what the problem might be?

Thanks.

 

Check your coolant level, I bet it's low. If it is, top it off and then warm the engine up and check for leaks. Look closely for a hairline crack in one of the plastic reservoir tanks or a pin hole in a hose or around the pump. Some place where you are loosing coolant slowly.