Make sure the vehicle has coolant. The Titan radiator cap had issue with it not sealing properly and cause coolant to escape. If the over flow tank is low, fill it.
If there is sufficient coolant, make sure the air mix door is moving when comanded to do so.
It could possibly also be an air pocket in the heater core. Flushing the system and raising the front of the vehicle when refilling should help.
Lastly, the thermostat stuck in the open position will cause this issure.
Answers & Comments
Make sure the vehicle has coolant. The Titan radiator cap had issue with it not sealing properly and cause coolant to escape. If the over flow tank is low, fill it.
If there is sufficient coolant, make sure the air mix door is moving when comanded to do so.
It could possibly also be an air pocket in the heater core. Flushing the system and raising the front of the vehicle when refilling should help.
Lastly, the thermostat stuck in the open position will cause this issure.