How to Fix 5 Common Car Heater Problems

cold person in jacket in car blowing on hands

It's 10 degrees outside, all you want to do is give up on being an adult and crawl back in bed, and low and behold, your car's heater is dead, and inside the cabin is colder than your freezer.

Car problems are never welcome, but a heater failure in the middle of winter is more than an inconvenience-it can put your and your loved ones in serious danger or, at the very least, turn a pleasant drive into an absolutely miserable experience.

A car heating system is a relatively simple affair, reminiscent of a refrigerator or an air conditioner turned inside out.

At its center is a heater core, a radiator-like setup located at the back of your car's engine compartment. The heater core is connected in line with the car's radiator, coolant hoses, and cooling passages. As the hot coolant runs through the core, a heater fan blows air across the fins, heating the air and moving it into your vehicle's interior.

A problem anywhere in the cooling system, failure of the water pump, or the lines and passages connecting the radiator and heater core will affect the heater's ability to keep your car at a comfortable temperature.

Watch for these signs of creeping up heater system problems:

  • A sweet, cloying smell from the heater vents. This is a sign of a coolant leaking from the system and burning up on the exhaust or hot engine block. Since circulating coolant is the core part of your car's heat and air conditioning systems, a leak is a sure sign of impending problems with your heater system ( and, more likely than not, an overheated engine).
  • Puddle of coolant underneath the car. These are easy to confuse with drips from the air conditioning system. Rubbing your finger in a drip-coolant will feel slick. Oily water drips from the air conditioning are just condensation and will feel like water. Make sure to clean up coolant leaks as soon as possible-animals are attracted to the sweet taste, and the main ingredient in coolant, propylene glycol, is extremely poisonous to them.
  • Car windows fogging up can be a sign of a coolant leaking inside the cab and evaporating or fan failure in the heater system. Touch the fog on the window-if it feels oily, it's most likely coolant. Run your hand above the vents leading to the window glass -if you're not feeling a strong stream of air while the heater is set on window defrost, your heater fan is gone.
  • Cold cabin after the car is properly warmed up(the thermostat does not start circulating coolant until 185-195 degrees Fahrenheit) is a pretty obvious sign of a problem in your heater system. Unfortunately, with outside temperatures always changing and fluctuating, it's hard to gauge the system's efficiency without paying deliberate attention.

Once you've determined that your heating system is having issues, it's time to roll up your sleeves and dive into diagnosing the problem. Start with checking for:

1. Lack of Coolant

Since your cabin's heat is directly derived from the scorching hot coolant used to cool the car's engine, a drop in the amount of coolant will directly affect the amount of heat in your car.

The cooling system is a closed system with no perceptible loss of fluid under normal circumstances. If you are seeing your coolant levels drop-you have a problem.

Plus, a coolant leak means air is getting into the system through the hole fluid is leaking through, lowering the boiling point of coolant and reducing its ability to do a good job protecting your car's engine from overheating.

Start with checking the coolant levels in the overflow coolant container.

To locate the coolant overflow bottle, follow the small rubber hose from the radiator filler neck to a plastic overflow container, usually found on the right or left side of the radiator, connected to the frame.

You'll see two marks on the side of the see-through plastic overflow container, the lower corresponding to the minimum acceptable coolant level and the upper with the maximum level the system can deal with before over-pressurizing and causing leaks.

Make sure to wait at least 20 minutes after driving the vehicle-as coolant heats up, it, like most other liquids, expands (the situation "overflow" reservoir is designed to compensate for) and will show an incorrect reading when hot.

A drop in coolant level usually points to a leak from a hose, connection, or gasket. Anything made of rubber will deteriorate, become brittle, and break, given enough time.

Start by checking the hoses themselves, looking for wetness and discoloration. Squeeze rubber hoses with your hand-if they feel soft and brittle; it's time for a replacement. Look at the connections between the hoses and metal parts of the engine and radiator-the clamps holding the hose in place loosen with time. A quick tightening might be enough to solve the problem.

Check for the signs of coolant leaks through the gaskets of the engine itself (head gaskets- the set seating between the cylinder block and the head of the engine). They're often hard to spot since the only time most engines will leak is when they're hot, and the metals of the engine expand.

A coolant leak from an engine gasket is a very serious problem, requiring a disassembly of an engine and quite a bit past the scope of this article. If your car's head gaskets are leaking, the best thing you can do is tow it to a reputable shop as soon as possible.

hands using coolant measuring tool to check car heater

2. Coolant not Circulating ( Thermostat or Water Pump Failure)

An engine in good shape should run at a temperature of right around 200 degrees Fahrenheit. To keep it nice and cool, the whole cooling system should be able to circulate the coolant, dissipate heat through the radiator, and, if you're using your heater-move some of the engine-produced heat into the cabin.

A thermostat is a temperature-reactive door that keeps the coolant from flowing into the radiator until the engine warms up to operating temperature.

It allows the engine to warm up faster so you can get on the road in minutes instead of an hour. A thermostat is also the reason why you do not see any warmth from the heater vents in your car until the engine starts to warm up while closed, it's keeping the hot coolant from reaching the heater core.

Thermostats are usually pretty foolproof, but given enough time and wear, they will start sticking. It will stick either in a closed position, keeping coolant from the heater core, or in an open, not letting the engine properly warm up.

Checking and replacing a thermostat is a pretty easy process.

Start with draining the coolant. Once done, trace the upper radiator house( it'll be one of the two big hoses connected to the radiator,1 1/2 to 2 inches wide) to the engine. The thermostat housing is the point where that hose connects to the engine, and it'll be connected to the engine block by two bolts.

Once the bolts are unscrewed, the thermostat housing/hose connection point will come off the radiator body, allowing you to simply pull the thermostat out.

Test the thermostat by placing it into a pot of cold water and heating it until it boils. As the water gets close to boiling point( 212 degrees Fahrenheit), the thermostat should open. If it fails to do so, trash it and replace it with a new one.

The water pump is responsible for circulating coolant through the radiator and cooling passages, cooling the engine and delivering hot coolant to the heater core. It's mechanically driven by the engine itself and, like any other part containing metal and moving parts, wears out over time.

Most modern water pumps can go for over a hundred thousand miles before needing a replacement( water pumps are rarely rebuilt since the cost of parts and labor is usually higher than a new part), but sooner or later, you will have to deal with a premature failure. With the water pump down, there is nothing to propel coolant through the system, resulting in no heat going to the cab and, shortly after, an overheated engine.

3. Heater Core Clogged

The walls of the coolant passages inside the engine oxidize and rust, and the rubber on the inside of the coolant houses deteriorates with time, adding up to debris that sooner or later will become a problem, plugging up the coolant passages in the radiator and heater core.

Some of the most common signs of debris buildup are the engine running hotter than normal and less heat coming out from your heater. Cooling system clogging up is a gradual process, and if you pay attention to your car's performance, you'll be able to catch it before serious issues develop.

To clean out debris from your heater core and your cooling system, start by draining the radiator. Once the cooling system is empty, fill it with distilled water and your favorite radiator flush product, close the radiator, and run the car for 10 to 15 minutes while keeping an eye on the temperature gauge to avoid overheating.

Running the car will move the debris into the water. Wait for the car to cool ( remember-radiator is pressurized when hot, and opening it can result in pretty bad scalding) and drain the water from the cooling system, removing the accumulated debris with it. Close the drain and refill the radiator with coolant and distilled water mix or premixed coolant.

4. Heater Blower Failed

If the air by the heater vents is warm, but there is no warm air moving into the cabin-there is a very good chance the blower fan your heating system uses to move warm air has failed. The heat is being produced by the cooling system and heater core, but without the fan blower running, none of it is getting into the cab, where it's needed.

Heater fan blower malfunction can be the result of the following:

Blown heater fuze is a perfect place to start in your diagnosis. Look up which fuse correlates to your car's heater fan motor in the manual or on the diagram inside of the fuse box's cover. Pull the fuse out and check to see if the filament has snapped. Replace it with the same voltage fuse if the fuse is blown.

Once you're sure the fuze(the quick and easy fix) is not at fault, it's time to look at blower resistors. Your car uses a series of resisters to control the speed with which the heater fan blows, putting a sizable amount of strain on them as the heater runs.

A resister burning out is a relatively common problem if you live in colder climates and use your heater for months at a time(as many of us in the frozen north do). A tell tail sign of a resister problem is the gradual failure of the heater to work on certain speed settings, with the heater failing at more speed settings failing as time goes by.

To repair the issue, all you have to do is replace the resister module ( a part easily found at most auto stores and usually priced at less than $25). You'll find the module next to the heater blower motor beneath or behind the dashboard. Disconnect it and remove the two screws holding it in place. Install the new module.

hand in front of car HVAC vent

5. Failed HVAC

If you remove your dashboard, you'll discover a maze-like HVAC system moving hot air from the blower to wherever you're ordering it to use an intricate system of vacuum-operated doors and passages.

A blender door stuck, closed, or misaligned is enough to negate all efforts by the heater core and blower motor to keep you warm. The cause can be vacuum related or the untimely demise of the actuator motor. Start with looking under the dash for an actuator motor at the end of the heater ducts. The actuator motor controls the closing and opening of the passages by eliminating (restricting)the vacuum to the door.

Grab a flashlight and an assistant and watch the actuator as your assistant switches the area warm area that should be directed to (defrost, lower, front, etc.). If you're not seeing any movement, it's time to replace the actuator.

To replace the actuator, remove the glove compartment or the lower dashboard cover(depending on the manufacturer and model) and unclip the actuator from the wiring. Unscrew the bolts holding the actuator in place and replace it with a new one. Clean the wiring clip, dab a touch of electric grease on it, and move the mixer door back and forth a few times before connecting the actuator to ensure it's not stuck.

Taking your car heater from freezing cold to subtropical comfy really doesn't take much besides a bit of know-how and some elbow grease(whatever elbow grease is). Your car's heating system is one of the simplest parts of your vehicle, fixing which is fully within the capabilities of a DIYer with a will to try and the gumption to figure things out.