My car is warm then when I turn on the heat the car blow out hot air when the car is move but when I am at a red light or stop sign the heat blow cold air in which the car is already warm for over a hour. What the problem? Heater core?

 

First, check the coolant level in the reservoir and/or radiator when cold.

 

It above the min. Should I refill it up to max or should I just flush the whole system?

 

When cold, open radiator to be sure that it is filled to the top and this will verify that the overflow system is working properly and the coolant in the radiator is to capacity.