2. Check the thermostat. The thermostat regulates the flow of coolant to the engine and heater core. If the thermostat is stuck open, the coolant will flow too quickly through the engine and not have enough time to heat up. This will cause the heater to blow cold air. Check the thermostat by removing it from the engine and placing it in a pot of boiling water. If the thermostat opens, it is good. If it doesn't open, replace it.
3. Check the heater control valve. The heater control valve controls the flow of coolant to the heater core. If the valve is closed, the coolant won't flow to the heater core and the heater won't work. Check the heater control valve by turning the heater on and feeling the hose that goes from the engine to the heater core. If the hose is hot, the valve is open. If the hose is cold, the valve is closed. Replace the valve if it is closed.
4. Check the heater core. The heater core is the part of the heater that actually produces heat. If the heater core is clogged, it won't be able to transfer heat to the air that flows through it. Check the heater core by removing it from the car and flushing it with water. If the water flows through the core easily, it is good. If the water doesn't flow through the core, replace it.
5. Check the fan. The fan blows the heated air from the heater core into the car. If the fan doesn't work, the heater won't be able to produce heat. Check the fan by turning the heater on and listening for the fan to come on. If you don't hear the fan, check the fuse for the fan and replace it if necessary. If the fuse is good, you may need to replace the fan.
