In order to tell if a heater core is going bad, look for the windows and windshield to fog up whenever the car is started. Learn about how wet carpet and the smell of mildew can indicate a bad heater core with help from an automotive technology professor in this free video on bad heater cores.
OK, this is a heater core that's been removed from a vehicle. First of all, the easiest way to tell that your heater core is bad is when you first start your car up, and your windows fog up, especially your windshield, you'll get all kinds of fog all over your windshield. That's one sure sign you ought to pay attention to. Some people ignore it, and they wait until somebody gets in the car, on the passenger side, especially if they're wearing flip flops, or no shoes at all, they're going to notice that they carpets wet because the coolant has leaked out on to the carpet. Third thing you're going to notice, if you're really in denial, when the mildew sets in, you're going to have an unpleasant odor in your car caused by the coolant remaining saturated in the carpet. Obviously to repair it, you have to replace the heater core, and a lot of these are not very convenient to get to, so you usually have to take the dash apart, and the whole plenum apart that houses the heater core, and the air conditioner evaporator, and remove this and replace the core. You can see where all this corrosion is where this core has been leaking right here, and again, a lot of these that leak on the seams, things like that, and these are where the hose connections where the heater hose connects up to the engine, and where the coolant goes through there. So, this is just nothing more than a heat exchanger, and it's just like radiator, except the heat is pulled to the passenger compartment, and it has a blower fan behind it, or it has outside air coming through it, and you can direct this down to the foot wells, or you can direct it to come through the dash vents, or you can direct it to come up through the defroster vents. A lot of times air conditioning compressors engage, too, to control humidity inside the cabin. And that's the heater core. So remember those symptoms, you start to see some fog on the windshield, when you start the car up in the morning, that's the first sign you need to do something. If you can't replace the heater core right away, you can bypass the heater core by taking these two hoses and just putting a splicer inside, splice fitting inside of it, join the two hoses together, and you can bypass the heater core. You won't have any heat, you won't have any defroster function, but at least you won't be leaking more coolant, doing any more damage until you can get this replaced. These can be very pricey because of the labor to actually get in there and remove this. Sometimes it can take the better part of a day to remove and replace one of these, that's when you know what you're doing.