Heater only blows cold air,all other heater functions operate, motor,air direction but no heat

BLEND DOOR ACTUATOR ISSUE.TEST WITH SCANNER TO MAKE SURE

underneath the dashthere's a black box that blows air out by your feet inside there is a small door like flap that cuts off air conditioning for heating controlled by your heating or cooling switch a lot of times this crap will break since it is plastic this will cause air temperature to stay the same no matter if you're on hot or cold it may be stuck closing off the heater pathway