my heater blows cold air and only heats when i am pushing the gas or driving on 1997 Mitsubishi Montero Sport

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got up the other morning and went to warm the truck and after 20 mins got in and the truck was not warm, had to leave so went to take the kid to school and the heater started blowing, it does everytime i am driving but idling goes back to cool air, what can be done to fix it?
i have checked the coolant levels and those are right and on the money
(1) Answer
Sounds like the blower fan has stopped working. The scoop will push air in when driving but, the fan does the real work. Check to see if it is running.