Dear Doctor: My father owns a 2009 Lincoln MKS with 48,000 miles. After 1.5 hours of driving with the outside temperature of 66 degrees with no a/c or heat on, I noticed fogging on the windshield (not heavy). It also happens in winter when the heat is turned on. Could this be a small problem now, or a big problem down the road? - Ursula
Dear Ursula: Today's vehicles are nearly air tight. Without air circulation - and under the right humidity conditions - windows will fog up. The HVAC system removes the humidity from the interior and draws in outside air in most cases. The air conditioning part of the system actually removes the humidity. Always make sure the carpet is dry, and remove the floor mats occasionally to let the carpet breathe. In most vehicles with automatic temperature control, the a/c compressor will come on anytime the ventilation system is on. In vehicles with a manual a/c, the compressor comes on when the system is set to defrost.