A foggy windshield happens when the air in the car is too humid, leading to condensation. The reasons can be natural, or they can be the result of some technical malfunctions:

A big difference in temperatures inside and outside. This problem is especially common in winter and late autumn.
Dampness in the cabin. If you get wet in the rain and get into the car, your windows will become foggy. It can also happen if you have water accumulate on the rugs due to wet and snowy shoes.
Heavy fog. This consists of small droplets of water, so it’s not surprising that the humidity in the cabin increases several times if there is fog outside.
Damaged sealing. Check to see how well the door seals perform their function and protect the car cabin from the outside humid air.
Dirt in the air filter. If it’s clogged with dust and leaves, it will impede the operation of the air conditioner.
Clogged AC drain pipe. Clean the drain hole, otherwise the air conditioner will not work properly.

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