“Even if you want warm air from your system, turning the AC on cools down the incoming air to just above freezing, but then it is reheated before it enters the cabin,” Fisher says. “This is done to remove moisture from the air, which otherwise would collect on cold glass, keeping you from being able to see out.”
When the outside temperature dips below freezing, the AC compressor will shut itself off even if the light stays on. Leave it on and forget about it. The AC doesn’t have to work very hard when the outside air is cool, so it doesn’t strain the engine or use much extra fuel.
