What Causes the Unpleasant Odor When Turning on the Car Air Conditioner?

[LeIris]

It may be due to a dirty car air filter. Here is relevant information about car air conditioning: Introduction to Car Air Conditioning: The car air conditioning system consists of a compressor, condenser, throttle element, evaporator, fan, and necessary control components, designed to regulate the temperature and humidity inside the vehicle, providing a comfortable environment for the driver. Working Principle: When the compressor operates, it draws in low-temperature, low-pressure gaseous refrigerant from the evaporator. After compression, the temperature and pressure of the refrigerant increase, and it is sent to the condenser. Inside the condenser, the high-temperature, high-pressure gaseous refrigerant transfers heat to the outside air passing through the condenser, causing it to liquefy into a liquid. When the liquid refrigerant flows through the throttle device, its temperature and pressure decrease, and it enters the evaporator. Inside the evaporator, the low-temperature, low-pressure liquid refrigerant absorbs heat from the air passing through the evaporator, causing it to evaporate into a gas. The gas is then drawn back into the compressor for the next cycle.

[DiKing]

I was driving when suddenly the air conditioner started blowing out a musty smell, really pungent. Asked a friend at the auto repair shop, and they said it's common when the cabin air filter hasn't been changed for too long, accumulating dust and even growing mold. Because the AC evaporator tends to condense moisture, mold thrives in damp conditions, and when you turn on the AC, it blows out the foul odor. It could also be due to rotting leaves or dead insects in the ducts. If not cleaned promptly, this smell isn't just unpleasant—inhaling too much might cause headaches or allergies. I often don't run the AC after parking in the rain, which makes it worse. Recommended going to a professional shop to replace the filter and get a full system cleaning, costing around a hundred bucks. Best to check the filter monthly and keep the car interior dry.