Just until a couple days ago my heater was working fine. Yesterday my brother pressed the defrost button which made the a/c came on. I quickly turned it off and switched it to 90 degrees and it just stayed cold... I thought it might be a one time thing but when i turn the car on in the morning at got it to running temp, I let the car sit for 10 mins and the fans dont come on. Only when I switch it to under 90 degrees...can someone help plz?

I check coolant and it was filled, check the upper hose it was getting hot, not overheating. Resetted ecu. Still the same. Is it thermostat or would you think its a heater core or something else?

BTW I have the climate control not the knob where u switch red and blue. Thank you