mastercraft

I think you may have the heater core air flow door is always open. you may want to try "manual" instead of "auto" control. simply, turn the all control off once and turning only the fan. and change the temp if this helps any. if helps, the auto mode control is broken. if this is not helping, you may need to remove the small panel by paasenger side at bottom(just remove one plastic screw and pull out.hard.) so you can check the heater door is moving correctly back and forth all the way by changing the temp.