Symptoms of a failing Heater Core
Leaks inside the car
Cars use a lot of liquid, so leaks aren’t that uncommon. The heater core, however, is the one component in your car that will leak inside of the car.
You’ll likely notice it as a puddle underneath the dashboard, though you might see it slowly dripping from the dash as well. Either way, that’s a sign that your heater core is having a tough time.
Similarly, you may not notice the sight of the liquid, but you might notice the smell of it. If you smell coolant in your car, it’s probably the heater core.
Heater isn’t working
Your car’s cabin heater depends on the heater core in order to work. Without the heater core, there’s no way for the climate controls to actually get hot air. So, if the heater core fails, your car won’t have any hot air, or perhaps will have only a tiny amount of hot air.
Fine in July. Not so much in January.
Windows are fogging up
Your windows may fog up for two reasons if your heater core fails. First, the car often relies on hot air when defrosting the windows, so it will struggle without that air. And second, the leaking coolant can cause a slight steam that will fog the windows to the point where you may not be able to see out of the windshield.
Coolant levels are low
Look, I’m not going to tell you how to spend your time. I’m just going to tell you that it wouldn't hurt to take a few minutes to check the fluid levels in your car. You might spot small issues before they become big issues.
If you notice that your coolant level is low, you probably have a leak somewhere. That leak may be from the heater core.
Engine is overheating
Or, you can just do it the less responsible way, by not monitoring your coolant level, and just waiting for your engine to overheat. Your choice.