It is cold outside and hopefully your heater is working as it should. Heaters work off the warm air absorbed by the radiator from the warmed-up engine. If your cold-day car ritual is to turn up the heat and blower right after you turn the ignition switch, all you’ll get is cold air. Only once the engine warms up will the passenger compartment warm up as well. So what is the cause if that warm air never arrives? Below is a laundry list of possible causes as to why you may be left sitting in the cold.
Coolant is low. If you’ve just recently changed the antifreeze, check the coolant level in the radiator to see if the radiator is full. An air pocket in the heater core or hose may be interfering with the flow of coolant through the heater core. You should also check for obvious leaks and have them repaired as necessary.
An open thermostat or one that’s too cold for the application.
A defective heater control valve. This valve allows coolant to circulate through the heater core even when the heater isn’t being used.
A plugged heater core. Accumulated crud in the cooling system may plug the core and block the flow of coolant. The only cure here is to replace the heater core. To prevent the problem from reoccurring, the cooling system should be flushed and refilled with a fresh 50/50 mixture of antifreeze and water. Distilled water is best since it contains no minerals.
An inoperative airflow control or inlet door in the heater ducting or plenum. If the defrosters aren’t working either, you’ve found the problem. Sometimes all that’s needed to fix the problem is to reattach or repair a loose cable or vacuum hose. On vacuum-actuated systems, however, the vacuum motor or control switch may be defective and require replacement.
A defective blower motor. If the blower motor doesn’t work (no sound/no air), the motor may be defective. Or, there may be an electrical problem such as a blown fuse, defective power relay, heater switch or resistor, or loose wire. Remember a blown fuse is a symptom not a cause.
A pinched or kinked heater hose. In rare instances, you might even find misrouted hoses if somebody worked on the cooling system recently.
A weak water pump (one with badly eroded impeller blades), or one that doesn’t turn fast enough because of a slipping drive belt.
Radiator overcooled by an electric cooling fan that remains on all the time, or a clutch fan that’s locked up. Excessive fan “roar” at highway speeds would probably be noticeable.
Dirty or blocked cabin air filter/s. You should check your cabin air filter if there is no other reason why your cabin should be fogging up, your defroster and/or heater is working inefficiently or there is a musty, stale smell in the cabin. (NOTE: there may be larger issues at play here than just a dirty cabin air filter, and you should have a certified mechanic investigate).