* Low Coolant Level: The most straightforward cause. If the coolant level is low, the system can't effectively transfer heat from the engine to the radiator. The engine overheats, and since the coolant isn't circulating properly, the heater core (which uses coolant to heat the air) won't get hot enough to produce warm air.
* Water Pump Failure: The water pump circulates the coolant. If it fails, coolant won't move through the engine, radiator, and heater core. This leads to overheating and cold heater air.
* Thermostat Stuck Closed: The thermostat regulates coolant flow. If it's stuck closed, coolant can't circulate freely, causing the engine to overheat and the heater to blow cold air.
* Clogged Radiator or Heater Core: Blockages restrict coolant flow. This can be due to debris, corrosion, or a build-up of sediment. The result is similar to a failed water pump: overheating engine and cold heater.
* Faulty Radiator Cap: A faulty radiator cap can prevent the system from maintaining proper pressure. This can lead to coolant boiling over and inadequate circulation, resulting in overheating and cold air from the heater.
* Head Gasket Failure (Severe): In a severe case, a blown head gasket can allow coolant to leak into the engine's cylinders or vice versa. This can cause overheating and a loss of coolant, leading to cold heater air. Often, a head gasket failure will also manifest with white smoke from the exhaust, milky oil, and low compression in some cylinders.
In short: While a low coolant level is the simplest explanation, the underlying cause could be more serious. It's crucial to *not* drive the car until the problem is diagnosed and fixed by a mechanic to avoid costly engine damage. Continuing to drive an overheating car risks warping the engine block or cylinder head.
