The heater core is actually part of the cooling system. As coolant circulates through the engine, some of that hot coolant passes through the heater core in the dashboard. As you use the temperature controls on your dashboard, your blower fan then moves the warm air to where you need it, for defrosting your windshield or heating the interior.
When everything is working properly, you have precise control of how warm your vehicle gets. But when you have a heater core problem, you lose this control. That’s a big inconvenience, and can even be dangerous if your windshield is fogging up or you’re stranded in the cold.
Heater core problems are unique, and not in a good way. The heater core is virtually the only item in your vehicle that can leak into your interior when there is a problem. All other components that are prone to leaking usually drip onto the ground. So heater core issues can be messy to deal with.
Most heater core problems are the result of poor cooling system maintenance. Flushing and replacing your coolant as specified in your owner’s manual can go a long way toward preventing issues caused by the buildup of rust particles, dirt and depleted anti-corrosion ingredients in the fluid. Your heater core, with its narrower passages, is one of the first items that can be harmed by this gunk.
There are several telltale signs that you’re experiencing a heater core problem:
Your vehicle is running fine, with the temperature gauge reading normal and no other cooling system problems. But when you turn on the heat, it never warms up.
Solution: First, rule out a blower fan malfunction by checking the fuse, the circuit and the fan itself if necessary. If that works, you can test the blend door and cabin vents. Start the car, let it warm up and then turn on the air conditioning full blast — you should feel the cold air. Now turn the temperature to full heat. You should quickly notice the difference and feel the hot air. You can then go through the venting options, including defrost and floor. If one (but not all of these) isn’t working, you likely have a blend door or vent problem.
If none of them work, you can also check the cabin air filter, which can get filled with dust and dirt, blocking airflow through the heating system. If the filter is OK, you may have a clogged heater core.
The heater core is buried deep behind your dash, and requires a lot of disassembly to get to. We recommend that you take your car to a mechanic for any heater core repairs, as it’s possible to do more damage to the car if you don’t have experience in this kind of work.
It might be possible to flush out the heater core’s passages with either a water hose or air pressure, but many modern heater cores have plastic lines and tanks so this can be risky. Don’t exceed the pressure stated on the radiator cap when flushing. Another possibility is that the exterior fins of the heater core are clogged with junk sucked in through the air intake. Cleaning the fins off may fix the problem if there are no other issues.
If this doesn’t resolve the problem, a replacement heater core will be necessary.
Additional reasons your heater isn't working
As you drive, you can smell the sweet, fruity scent of coolant inside.
Solution: Your heater core may be in the early stages of failure. A leak as small as a pinhole in your heater core can spray a fine mist of coolant into the interior, and you’ll smell it. Avoid stop-leak products, as they’re at best a temporary solution and can cause other problems instead. It’s better to call your mechanic and have this taken care of now, before it gets worse.
This is a more extreme version of the coolant smell scenario. The amount of coolant spraying out of the heater core is enough to form a mist that coats your windows and is difficult to wash off. This is bad — not only because it blocks your vision, but also because breathing ethylene glycol is harmful to your health.
Solution: Call your mechanic, roll down your windows, and get your vehicle in for repairs immediately.
You may notice coolant dripping down from under the dashboard, or there may already be a big, damp coolant stain on your front carpeting.
Solution: Check your coolant level and top it up as necessary. If coolant is leaking into the interior, your engine is in jeopardy. A low coolant level can cause your engine to overheat, which could lead to a big repair bill. Call your mechanic immediately and get the leak fixed. Once the repairs are completed, have your carpet professionally cleaned or replaced in order to eliminate the coolant smell.
If you have a leak anywhere in the cooling system, including the heater core, it’ll cause a couple related problems. Your car will lose a significant amount of coolant, and eventually your temperature gauge will run higher than normal.
Solution: Top off your coolant and take your car in for inspection right away. A coolant leak won’t fix itself — it’ll only get worse. Again, if you drive without enough coolant in your car, your engine will begin to overheat. This can quickly ruin the engine.
Buried deep within your dashboard, the heater core requires the disassembly of several other in-dash systems before you can access it and make the necessary repairs. If you need a new one, get an estimate, as this is both a complex and time-consuming job. You’re usually much better off having it replaced instead of attempting a repair — you only want to have to do this once.
The heater hoses should be inspected when replacing a heater core. Problems with the heater hoses can cause similar problems to a failing heater core and are less expensive to correct.
Some cars will need to have the air conditioning system recharged, which can increase the cost of repair. Another problem to watch out for is worn engine mounts. If the mounts are worn, they can allow the engine to move more than normal, which can damage heater core connections. Worn mounts should be replaced.
If you don’t have the time or the money to repair your heater core immediately, there’s a possible short-term solution. You can disconnect both heater core hoses where they enter the heater core, and then connect them together using a coupler. This effectively bypasses the heater core and usually stops the leaking. Again, this is a temporary fix, and will leave you without heat and your defroster.
For this reason, this fix is best used when the weather is warm — it’s not recommended during the winter. If your heater core fails when it is cold out, you’ll want to have it fixed right away.
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