The heater functions with the help of a healthy cooling system and the heater controls inside the passenger compartment. Depending on the symptoms, there are various reasons why your heater might not be working.
Engine temperature gauge is normal, but I have no heat!
First, check your engine’s coolant level. If the coolant level is low, air may become trapped in the heater core. Smooth coolant flow through the heater core is critical for heat output.
While the vehicle is running, heat produced by the engine is absorbed by the engine’s coolant and flows through the heater core. The heater core is a little radiator usually located behind the dashboard in the passenger compartment—it is the heat source for your vehicle’s climate control system.
Always exercise extreme caution when checking anything in the engine compartment as many engine components can be hot—unintentional contact with rotating components may cause injury.
Debris may restrict the heater core, particularly if excessive amounts of a "stop leak" product have been used to address a coolant loss problem. Locate the heater hoses that carry coolant to the heater core—they will be at the rear of the engine compartment and are approximately an inch in diameter. With the engine at normal operating temperature, feel for a noticeable temperature difference. With the temperature controls set to the full hot position and the heater blower off, the two hoses should be the same temperature. If there is a significant temperature difference, this may indicate a plugged or restricted heater core. Flushing the cooling system and disconnecting the hoses that are attached to the heater core in order to inspect coolant flow through the heater core might be a worthwhile maintenance.
Does your vehicle have a heater control valve installed inline in a heater hose? Some vehicles (usually later models) have a heater control valve either operated by a cable attached to the heater controls on the dash or controlled by a vacuum signal. (Vacuum-operated heater control valves are more common on cars with climate control heating/air conditioning systems.) If equipped, verify the heater valve is not blocking coolant flow to the heater core—you may need to have the engine running while checking this. Have a friend operate the temperature control and verify the valve’s operation.
Engine temperature gauge is higher than normal, but I have no heat!
If your temperature gauge is showing higher-than-normal temperatures, there could be a restriction in the cooling system. This is often caused by a defective thermostat that is stuck in the closed position or other restrictions. Be careful not to overheat the engine and don’t wait to take care of cooling system problems!
The air is cold when I stop, but warms when I start driving again!
A healthy cooling system is critical to heater operation—"fresh" coolant should be constantly flowing through the heater core. If the heat increases when you start driving after a stop, an inefficient water pump might be the problem. As the impeller in the water pump wears, it becomes less efficient when the engine is idling. The problem might also be a restriction somewhere in the cooling system. You may also notice the temperature gauge reading higher than normal. Careful diagnosis is needed in this case—be careful not to overheat your engine!
The engine takes forever to warm up, or never warms up!
If your car is slow to develop adequate heat when the engine is started from cold—or the engine takes a long time to "warm up"— take note of the temperature gauge. The engine should reach operating temperature (approximately the middle of the gauge) after about six to eight minutes of running. If the temperature gauge needle always remains in the bottom third of the gauge, it is likely that the cooling system thermostat has failed.
My cooling system is fine, what else could it be?
If you are certain that the cooling system is functioning normally, it is possible that the source of the problem is in the heater controls.
As mentioned earlier, some cars have a heater control valve and many manufacturers have switched to temperature blend doors inside the heater/HVAC housing. Poor heater operation may be due to heater controls that are not functioning properly. Climate systems usually operate using cables, vacuum, or electronic signals and actuators.
Hot/cold temperature blend doors commonly fail on many new vehicles. Depending on the model, it may be the door that fails, the actuator that moves the door, or the signal to the door (whether it’s an electrical signal, cable, or vacuum). With temperature blend doors, coolant is always flowing through the heater core. Air in the heater housing is diverted around the heater core in the heater housing. A defective blend door may not allow the air to pass through the heater core, which prevents the vehicle from warming up.