I live in Minnesota with my stock 2006 Mazda 6 four-banger with 190,000 miles. The heat is only hot when I am driving, when I stop for a red light, the air blows cold through the vents. The opposite happens when I have the Air Conditioning on. The air only blows cold when I am driving. When I am at a red light, the air is not cold, it is warm coming through the vents. The heat gauge does not show the car is overheating. I replaced the anti-freeze coolant but that made no difference. What could the problem be?
For your heater problem, also check operation of the temperature blend door, especially if vacuum-operated. (Engine vacuum is highest at idle.) As already mentioned the AC problem is probably not related and could be due to a number of different causes.
You have two different problems. When the heat doesn't work while stopped there could be air trapped in the cooling system, low coolant level, a faulty radiator cap or something along those lines. When the AC doesn’t work while stopped but only blows cold air while driving, the cause may malfunctioning condenser fans, low refrigerant level, stuck compressor clutch, and/or dirty compressor. I'm not aware of any single cause for both issues. Good luck.
