2004 Ford Explorer heat problems

Hi, working on a 2004 ford explorer, has rear heat no front heat, pulled out dash though the blender door was not working or blender door motor bad, but both is good, would you think maybe the heater core is blocked, also checked all actuators to make sure they worked, it has dual heat front and rear, back heat works great, but front heat is not warm, checked all actulators and blend door and motor all is working good, das settings work like they suppose to, but no heat just blows cold to luke warm air, would the next step be to check the heater core and see if it is stop up, appreciate any advice you can give