A heater core is like a miniature radiator inside your car. Unlike your radiator, the idea of the heater core is to release heat into your car as heated coolant passes through it. The heater core is normally located behind the dash. Most often, it is on the passenger side of the car. With 5 years of mechanic experience, I have found a few simple clues that let you know if the heater core is failing.
The heater core may be bad if the car cabin no longer heats up properly in cold weather.
Like the radiator in your car, the heater core can become clogged. When this happens, coolant will not circulate through it enough to provide adequate heating for your car. You will notice this most in very cold weather when the heater may have to work at near-capacity to warm you up. If the cabin of the car never seems to get to a comfortable temperature or your feet stay cold on longer trips, the heater core may be the culprit. The only real solution for this problem is to replace the heater core.
A failing heater core may leak onto the floorboard on the front passenger side of the vehicle.
Since the heater core is generally on the passenger side of the car, this will be where leaks will be seen. Most of the time, you will either notice the smell of coolant in the vehicle, passengers will complain of warm liquid dripping on their feet, or wet spots will stain the carpet on that side of the vehicle. This leak can show up whether your heater is on or not. For a temporary fix, you can disconnect the heater hoses from the heater core and clamp them together so that the coolant circulates without going through the heater core. Replacing the heater core is the fix for the long term.
The connection to a heater hose may degrade and leak.
Actually, a leaky cooling system from any source can cause your heater to keep working. However, if the leak is outside your vehicle but near the firewall, one or both connectors from the heater core may be corroded. When this happens, the leak will be between the end of the heater hose and the firewall of the vehicle. Once again, you cannot permanently fix this without replacing the heater core. Because of pressure in the cooling system, it is not likely that any of the products to repair metal will work. You might be able to get someone to weld it for you, but do not hang on to too much hope for this work.