Q: Causes for heater to blow cold air

Hi...if you are referring to the passenger compartment heater, there are two possible causes of insufficient or no heat. The passenger compartment heater, also known as the "heater core", is essentially a mini radiator and the fluid running through this radiator is the engine coolant. The flow of the coolant into that radiator is controlled by a "heater control valve" that is mounted on the engine compartment firewall. If the valve is stuck in the closed position, the hot engine coolant will not flow to the heater and hence there will be no heat. So, you want to check for free operation of that heater control valve first. The second possibility is a problem with the cold/hot blend door in your car’s HVAC system. Similar to what would happen with a stuck heater control valve, if the blend door is stuck on cold, and thus blocking air flow over the passenger compartment heater, you will not get heat. YourMechanic can readily diagnose your heating system and repair either of these faults for you and would be pleased to do so if you contact us. Thanks for your question.