On a 92 Nissan Sentra could it be stuck shut and not letting heat into my ducts? I have no hot air. I have no heat, have checked all the hoses and all are hot going to the heater core. Have changed the Temp. Gauge, checked to see if anything was clogging the air flow. Could the Heater case door be stuck shut? If so where is it located? I have air flow, just no heat.