Crash Course in Car Heater Operation
Most cars on the road have water-cooled engines, and their heating systems work on the same basic principle. Hot coolant from the engine passes through a heater core, which looks and functions like a small radiator, and a blower motor forces air through it. The coolant then heats the air, and the air, in turn, warms the interior of the vehicle. This is the reason that it takes a while for heaters to start blowing warm air. Until the engine warms up, there’s no heat for the heater core to extract. It’s also the reason that a plugged heater core, stuck thermostat, or air in the cooling system can cause a car’s heater to blow cold.
Car Heater Blowing Cold Due to Cooling System Issue
The four main cooling system problems that can cause a heater to blow cold air are:
Stuck thermostatAir in the cooling systemPlugged heater coreCoolant not flowing through the heater core
It’s a little more complicated than that in practice, but these are the most common heater issues that you’ll run into.
Stuck Thermostats
Thermostats are valves that open and close depending on the temperature of the coolant. As the engine warms up, they stay closed until the coolant in the engine reaches a specific temperature range. If they fail to open at that point, the coolant won’t circulate properly, the engine may overheat, and you might experience a problem where the heater blows cold air. When a thermostat sticks open, it can prevent the engine from warming up properly or prolong the warming-up period. If the heater is blowing lukewarm instead of cold air, a stuck open thermostat could be the cause.
Air in the Cooling System
Another common problem occurs when air gets into the cooling system. Since the heater core is often the high point in a cooling system, air can move into it and become trapped. If that’s the case, the air bubbles must be flushed out to fix the problem.
Plugged Heater Core
Plugged heater cores can also cause a car’s heater to blow cold. The best way to check for this is with a non-contact thermometer. You use it to check whether coolant is flowing through the heater core. If it isn’t, flushing the heater core often fixes the problem. Some vehicles have a valve installed in the heater core inlet line that is operated by vacuum or a mechanical cable. If that valve is stuck closed, that’s another reason a car heater will blow cold.
Coolant Not Flowing Through the Heater Core
A heater core can become plugged in more than one way. When you hear about a plugged heater core, that usually means that corrosion or other junk has clogged up the internal tubes, and flushing will often clear it up. However, the fins of a heater core can also be clogged with lint, pine needles, and other detritus that manage to get into the heater box. The fix for this is to open or remove the heater box and clean the fins.
Other Reasons a Car Heater Can Blow Cold
Most of the reasons a car heater blows cold have to do with the heater core. Still, you can also have a mechanical, electrical, or vacuum problem. The specifics vary significantly from one vehicle to another, but most systems have a blend door that changes how air flows or doesn’t flow through the heater core. When a blend door is stuck, it doesn’t matter if the heater core is working perfectly. Since the blend door is stuck, the heater core is essentially bypassed, and you won’t feel anything but cold air. A blend door can stick for a number of reasons, and they don’t always stick the same way. It can be stuck open, resulting in all heat all the time, or stuck partially closed so all you get is lukewarm heat. A blend door can also be stuck because of mechanical linkage or a vacuum line coming off, a switch going bad, or several other reasons. If you suspect a blend door issue, the specific diagnostic procedure depends on how your vehicle’s heating system is set up.
