The heater core….sounds like something from a nuclear power plant but, in fact, it’s a vital part of your car. So, if it starts causing problems, this list of bad heater core symptoms should help you pinpoint the problem.

It is in charge of warming up the passenger cabin and providing that sense of coziness and comfort during cold days. Its role is so important that without it the driving experience simply wouldn’t be complete nor, in some cases, possible.

If you’re interested, you can read more about the definition of a heater core (provided by Wikipedia) by clicking here.




The first and most obvious bad heater core symptom.

You see, the heater core is an integrated (connected) part of the coolant system.

The same coolant that goes through the system is the one that goes through the heater core. Having this in mind, if the heater core is bad (meaning leaking) the coolant level will go down.

At first, it will be barely noticeable but as the leak worsens you’ll have to add coolant every now and then.

One more reason why you should make regular checkups a habit. You’ll be able to spot the problem on time for sure.




Adding a small amount of coolant once or twice a year is OK, but adding some every month or more means you have a problem.

Have in mind that coolant loss can be caused by other reasons like a punctured radiator, ruptured coolant hose, loose clamp, or else.

If the rest of the system after inspection seems OK, then you should check the heater core as soon as possible.




Loss of coolant will eventually lead to engine overheating if the problem goes unattended.

Simply, less coolant means the whole system is less efficient.

Overheating will happen in the final phase of the problem and if you pay attention, it will never get to this. In most cases, a bad heater core will start with a minor leak that can last for weeks if not months.

But if neglected, the leak may suddenly turn in to a coolant flood. Before this, you’ll probably notice the smell of coolant, higher reading on the temperature gauge, coolant vapors, or the wet carpets.

So plenty of time to react and notice bad heater core symptoms.

Try never to get to this phase as overheating has serious consequences to the engine. It can even get to, in more radical cases, a warped cylinder head or even an engine overhaul.

One more important thing worth mentioning on this part: you can drive with a bad heater core provided that you add enough coolant and the leak is not too big.

When the leak happens during the summer season, you can turn the heating regulator to cold. This way the heater core is separated from the rest of the coolant system and reduces or completely stops the loss of coolant.

This is good only in case of emergency and for short distances when you’re forced to drive the car and don’t need cabin heating.

In winter, you’ll have to solve the problem as soon as possible as the car will be pretty much undrivable.

If the overheating isn’t caused by a bad heater core, you can read this article about the most common reasons for an overheating car.




Now we come to one of the most noticeable bad heater core symptoms.

You see, lost coolant has to go somewhere.

In most cars, the heater core is located inside the passenger cabin (near or under the central console).

Because of this, the usual scenario is that coolant drips on to the carpets, floor insulation, and eventually penetrates to the floor panels.

Over time, everything will get saturated with coolant.

This symptom can go unnoticed for a long time, especially if you’re not a fan of cleaning the car interior. If you are, you’ll surely notice the wet underside of the car mats.

More on how to clean the car interior in a separate article which you can read by clicking here.

Besides that, if you rub the carpet underneath and get a greasy sweat-smell like film on your fingers, you’ll know for certain that you have a bad heater core.

Wet car mats and carpets can, of course, also be caused by water and snow getting in with your feet but these dry off pretty quickly and in most cases don’t have any odor. When there’s a bad heater core problem, the car mats will always be wet.

Noticing this symptom on time is very important. When the carpets and insulation get saturated with coolant the only solution is to dismantle parts of the interior and get everything that’s wet outside.

With most cars, this is a painstaking and time-consuming job involving things like taking of trimming, various easy-to-break clips, taking out the car seats, etc.

Added to that you have to thoroughly clean and dry up everything before returning.

Opposed to this, if you notice these bad heater symptoms on time, a simple dry cloth and a better vacuum cleaner can perhaps solve the problem.




It’s already been mentioned that the heater core is in charge of cabin heating.

The principle is that once hot coolant starts flowing through it, it emits hot air. At the same time, a ventilator located behind it blows air through the core and into the ventilation system.

So, if you sense that the cabin heating is weaker than usual or in worst cases non-existent, then you might certainly have a bad heater core.

At first, you’ll barely notice it, but as the problem progresses, it will become weaker and in the end, stop working.

This symptom appears in the latter stages of the problem. So even if you don’t notice the wet carpets or low coolant level, you’ll certainly notice the cold temperature inside the passenger cabin.




The heater core starts producing heat only when the coolant gets hot.

Most drivers know this and don’t expect the heating to start working until the engine temperature starts rising.

Usually, the moment the engine starts heating up, you can feel warm air coming out of the vents.

But if you feel no hot air is coming out of the vents even when the temperature gauge is showing normal engine temperature, then you may have a heater core problem.




Coolant has a specific sweet smell. If you’ve never smelled coolant, just take off the cap of the coolant bottle (when the engine is cold), smell it and you’ll know what I mean.

Once coolant starts leaking into the passenger cabin, the smell will be always present. Even when the engine is cold and you get in the car, you’ll probably sense it right away.

When the engine heats up, the smell will be even more noticeable. The reason is that the coolant is starting to evaporate.




This is one of the bad heater core symptoms that is perhaps the most dramatic.

As mentioned, once the engine starts heating up, the coolant will start to evaporate through a bad heater core. It will probably evaporate into the ventilation system.

If the ventilation is pointed towards the windshield (which it mostly is during the winter), coolant vapor will start to form a thin and greasy film or fog.

At first, while the leak is minor, it will be hard to notice. If you take a cloth and try to clean it, it will leave smears as well as a greasy stain on the cloth. It will be accompanied by a weak smell of coolant.

In the later phase, the foggy film will be present all the time. The moment you start the car, it will appear.

By this time, you must take action to solve the problem. Otherwise, you may end up with a very unpleasant scene.

In the final phase, a thick mist of vaporized coolant may suddenly start pouring into the passenger cabin accompanied by a strong smell of hot coolant.

In a matter of minutes, driving will become pretty much impossible. You’ll have to pull over, turn the engine off, and get out of the car as soon as possible.




Another tell-tale sign is a foggy cabin whenever you leave the car for a couple of hours.

If you have a bad heater core, once you stop the car, turn off the engine, and the ventilation system stops working, the coolant tends to evaporate and causes a foggy cabin.

Simply, a condensation effect happens.

Have in mind that this can be caused by other things; like water leaks into the passenger cabin so make a check for other possible problems.

Again, this is one of the bad heater core symptoms that are recognized by the sweet smell of coolant. So, a foggy cabin plus coolant smell probably means a bad heater core.




Besides leaking, a clogged heater core will also cause trouble.

Just like the car radiator, it is made out of a combination of aluminum mesh and small diameter tubes through which the coolant flows.

Over time, these tubes can get clogged for various reasons (using tap water instead of coolant, the build-up of filth inside the system, etc). When the heater core clogs up, it causes an increase of coolant pressure inside the system.

If not tended to it can cause engine overheating, coolant hose cracking, or else.

This is one of the bad heater core symptoms that is recognizable by a mild engine overheating as well as constant high pressure in the coolant bottle.

Of course, the coolant bottle is always pressurized, but when everything is normal, you’ll just hear a short ˝sssshhhhh˝sound when you open it as opposed to a strong hissing sound all the time.

These symptoms will appear when the cabin heating is on. When it isn’t, you probably won’t notice anything.

Needless to say, open the coolant bottle only when the engine has cooled down a bit, and be careful not to burn yourself.



Although this article will take you some time to read, pinpointing the problem in a real-life situation is a matter of minutes, if you have some experience.

It usually looks something like this: if you make regular check-ups on your car, you’ll notice a constant loss of coolant. You then check under the hood and if everything is OK, checking the car mats and carpets is the next step.

If they’re moist or wet, bingo! You’ve located the problem.

Another sequence of events is a constant smell of coolant in the passenger cabin accompanied by mild engine overheating. Then, you check the coolant level, after that the car mats and carpets.

So, in a matter of minutes, you’ll know what the problem is.

In the end: solve the problem as soon as you notice any of these bad heater core symptoms. Although this problem shows itself gradually and can be hard to spot on time, some caution and extra vigilance will help you before it’s too late.

Neglecting the problem will lead to loss of cabin heating or even worse, engine damage through overheating.

In case of a bad heater core, neglect is billed through driving a portable refrigerator in the winter, surrounded by clouds of coolant vapors while, at the same time, the engine is overheating.

Should be good enough reasons to take action on time.



Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com