2005 Mazda 6.
Heater blows hot air when gas is depressed (so if the vehicle is moving or if giving the vehicle gas while sitting at a stoplight). If I take my foot off the gas and stop, the air blowing out becomes cool within 5 to 10 seconds. Once I rev the engine or just start driving again the air becomes hot again within about 5 to 10 seconds. The temperature change is quite remarkable. The fan is blowing well the whole time. Any thoughts?

 

ya know, i noticed mine doing that today too. my heater was blowing heat when i was moving, but when i stopped at the stoplight, it was still blowing hot air, but not as hot as when i was moving..... did you find anything out?

 

Check the cabin filter behind the glovebox.

 

You might also want to check the coolant level.

 

ok, thanks. i just changed that filter. so it couldn't be that.. i just did an oil change and checked all my coolants. but it only did it that one day, so i think it's good now. it hasn't done it since. thanks though!