They say heater core, Ford dealer replaced it 3 years ago. Its leaking from the drain. I have luke warm air from vents. I was told to turn the temp knob from cold to hot a few times, I did and it got hot, also the leak from drain stopped. But this is happening every few months. If I smell anti freeze and look under my truck , I see the leak from the drain. when I say leak, its a pretty nice one. but I do the switch thing from cool to hot a few times, when its gets good hot air, I look and the leak stops. the dealer still says heater core. I want to think maybe blend door is sticking but I don't understand why the leak.
The heatercore is hot regardless of the position of the knob on the dash. The knob just controls if a small door covers it.
If the heater core was replaced with a cheap aftermarket unit it may have failed again. Otherwise it is possibly leaking at the connections where the rubber hoses are clamped to metal piping.
I recommend getting a small section of pipe that is the correct diameter to fit on the rubber hose. With the engine completely cool, you can disconnect both lines at the firewall. Install the small piece of piping between the lines and reinstall the clamps. This bypasses the heater core. If the leaks stops after this, the problem is in the heater core itself.
It's your heater core. The blend door only controls air flow inside the HVAC unit. The heater core bypasses the thermostat so that heat can be achieved as the engine warms, you don't have to wait for the thermostat to open (or engine to warm up) for heat. Also handy if the radiator fans stopped working or you're overheating you can turn your heat and fans to high and it will help reduce engine temp, as the heater core is a mini radiator inside the HVAC unit. On newer vehicles this requires quite a bit of labor to remove for some as the whole dash has to be removed or partially removed and just slid back to gain access to the HVAC unit.