* Low Coolant Level: The simplest explanation. If the coolant level is low, there's insufficient coolant circulating through the heater core, resulting in cold air. Check the coolant reservoir immediately. Low coolant can be due to a leak somewhere in the system (radiator, hoses, water pump, etc.).
* Thermostat Stuck Closed: The thermostat regulates coolant flow. If it's stuck closed, coolant won't circulate properly, leading to overheating and no hot coolant reaching the heater core.
* Air in the Cooling System: Air pockets can prevent proper coolant circulation. This often manifests as inconsistent heating or cooling, and can contribute to overheating. Bleeding the cooling system (a process to remove air bubbles) is often necessary.
* Faulty Water Pump: The water pump circulates the coolant. A failing water pump won't circulate coolant effectively, leading to overheating and a cold heater.
* Clogged Heater Core: While less likely to cause overheating on its own (overheating is usually the *cause* of cold air in this scenario), a severely clogged heater core *could* restrict flow enough to exacerbate an existing overheating problem. However, a clogged heater core would usually manifest as *weak* or *inconsistent* heat, not completely cold air unless the other issues above are also present.
* Defective Blend Door Actuator: Although less probable given the simultaneous overheating, a malfunctioning blend door actuator could be preventing hot coolant from reaching the heater core *even if* the coolant is properly circulating. This is more likely to result in only lukewarm air or inconsistencies rather than completely cold air.
Crucially: Ignoring an overheating engine is extremely dangerous and can lead to significant and costly engine damage. Do not continue driving the car if it's overheating. Have it towed to a qualified mechanic for diagnosis and repair. The cold air from the heater is a symptom of a more serious underlying problem.
