Question 1

Why do my car windows fog up and what clears them fastest?

Accepted Answer

Fog forms when moist cabin air meets glass that is below its dew point, and the water condenses out onto the surface. Which side of the glass it forms on tells you what to do, and the two cases need opposite treatments.

Fog on the inside is the common one. People are the main moisture source — breathing and perspiring add a surprising amount of water to a sealed cabin — along with wet coats, umbrellas, snow on footwear and damp carpets. On a cold day the glass is the coldest surface in the car, so that is where the moisture condenses first.

The fastest fix is counterintuitive to many drivers: use the air conditioning, even in winter. An air conditioning system is a dehumidifier. It cools air below its dew point, strips water out of it, then reheats it. Air conditioning plus heat plus the windscreen vents clears an interior fog several times faster than heat alone. Also switch off air recirculation, which traps the humid cabin air you are trying to remove.

Fog on the outside happens in warm, humid weather when the cabin is cooler than the outside air — often after running air conditioning hard. That one is cleared by the wipers, or by raising the cabin temperature slightly.

Persistent misting that returns immediately suggests a water source: a blocked air conditioning drain, a leaking door or windscreen seal, or a heater matrix leak. A sweet smell and an oily film on the inside of the windscreen point specifically at the heater matrix.

Keeping the inside of the glass genuinely clean helps, because condensation forms more readily on a greasy film.

Question 2

How does car air conditioning work?

Accepted Answer

Exactly like a fridge — it moves heat out of the cabin rather than making cold — and the same cycle also does something people do not realise it is doing: removing water from the air. The cycle. A

Question 3

Why does my car air conditioning smell musty?

Accepted Answer

Because water is condensing inside the system and something is growing in it. The smell — usually described as damp, mouldy or like old socks — comes from bacteria and fungi colonising the

Question 4

Why do my windscreen wipers smear instead of clearing?

Accepted Answer

Smearing is nearly always a contamination problem rather than a worn-blade problem, which is why new wipers sometimes smear straight out of the packet. A wiper works by dragging a thin, sharp rubber

Question 5

Why does a windscreen replacement now need a camera recalibration?

Accepted Answer

Because the camera for the car's driver-assistance systems is mounted on the windscreen, and moving it by a fraction of a degree moves where the car thinks the road is, hundreds of metres ahead. What

Question 6

Why have my headlights gone yellow and cloudy?

Accepted Answer

The lenses are plastic, not glass, and the outer layer has been degraded by sunlight. Almost every car built since the 1990s uses polycarbonate headlight lenses, which are light, cheap and far less

Question 7

Why does my car use more fuel in winter?

Accepted Answer

Cold weather reduces fuel economy through several mechanisms at once, which is why the drop is often larger than people expect. A reduction of ten to twenty percent in cold conditions is normal

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