Water leaks on driver side floorboard on 2004 Hyundai

During the summer, it is NOT the heater core unless your car is a sauna. If it is only on the passenger side, I'll also bet it only happens when the air conditioner runs. Look under the car around the firewall on the passenger side for a small tube hanging down. This will be the drain for condensation on the AC. Carefully poke a wire up this hose to clear the blockage.

If it is coolant? You have a bad Heater core. If it is just water? You have a bad Hood seal or a bad firewall seal.
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This can be caused by a couple different things. You could have leaky door seals or rubber water seals, or you can have a leaky air conditioner compressor that can leak.

Does it have a sunroof? if so check your drains which are located in the corners of the sunroof opening (glass will have to be open to see them) if the drains are plugged find a friend with an air compressor and a blow gun and gently blow air into the drains until all debree is cleared. if you DONT have a sun roof, check door seals for tears and also another check have someone spray the windsheild and check for leaks, you may have a windsheild seal that has let loose and will need to be resealed