What would cause a car heater to blow cold air?

1. Low Coolant Level: If the coolant level in your car's radiator or coolant reservoir is low, it can't effectively transfer heat to the heater core. Check the coolant level and add more if necessary.
2. Faulty Thermostat: A faulty thermostat may not open correctly, restricting the flow of hot coolant into the heater core. This prevents the heater from receiving enough heat.
3. Blocked Heater Core: Over time, the heater core can become blocked with debris, which restricts the flow of hot air through the vents. Flushing the heater core and removing any blockages may resolve the issue.
4. Air in the Cooling System: Air pockets in the cooling system can interfere with the circulation of the coolant, affecting the heater's efficiency. Bleeding the cooling system to remove air can solve this problem.
5. Faulty Heater Control Valve: Some vehicles have a heater control valve that regulates the flow of hot coolant to the heater core. If this valve is faulty, it may not open or close properly, restricting the hot air supply.
6. Faulty Blend Door: Inside the dashboard, a blend door helps mix hot and cold air to control the temperature. If the blend door or its actuator is faulty, it may not properly direct hot air into the cabin.
7. Electrical Issues: Electrical problems, such as a blown fuse, faulty wiring, or a malfunctioning switch, can disrupt the power supply to the heater fan or other components in the heating system.

If you have tried these basic checks and the heater is still blowing cold air, it's best to take your car to a qualified mechanic for further diagnosis and repair.