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When summer’s here, or really any time you want cool, dry air while driving, your car’s air conditioner (AC) is critical to your comfort and safety. Unfortunately, frozen AC lines can put a major kink in your road trip. Why are your car AC lines freezing up, and what you can do about it?

Usually, AC is one of those forgotten systems that you never really think about until it stops working. Sometimes there can be worn or unmodified AC systems that function for years without a problem. On the other hand, if your car has been in an accident, has ever had any AC components removed or had any service done, the situation is ripe for frozen AC lines.

What Causes Car AC Lines to Freeze Up?

Really, the only thing that should be in your car’s AC system is refrigerant and oil. To keep these things inside and others — such as air, moisture and dirt — out, the pipes, hoses and seals are specially designed to maintain a tight seal. The problem of car AC lines freezing up is caused by moisture that has been introduced into the system. At the expansion valve or fixed orifice tube, depending on the system, liquid refrigerant quickly expands into a gas, forcing the temperature to drop. Moisture in the system can freeze at that point, blocking refrigerant flow through the valve, and you’ll notice you have warm air coming out of the vents. As the valve warms up again, the ice melts and refrigerant flows, so you’ll get cool air again, but the cycle will continue.

Generally, the only way that moisture can get into the AC is if the system is opened up or if you have a leak. If there is a leak, air and moisture can be pulled in by vacuum on the low side of the system. Also, any time that the AC system is opened, moisture can get in. For example, if you use an R-134a top-off bottle to fill it up, a little moisture can get in when you open the valve cap and install the can. Similarly, if an accident breaks the system, such as the condenser or one of the lines, the open system can freely absorb water from the air. Normally, the receiver-dryer absorbs some of this moisture, but it only has so much capacity before it is saturated, leaving the rest to float about in the refrigerant stream.

What Can You Do About Moisture in the System?

If you notice your car AC lines freezing up, you have to get rid of the moisture that’s causing it, starting with a full leak check. Recover the refrigerant from the system and make any necessary repairs to parts such as O-rings, hoses, lines or evaporators. If the system has been opened for more than a few hours from an accident, damage or corrosion, the receiver-dryer absolutely must be replaced. It can become saturated when left exposed to the air. Alternatively, if the system has only been open a few minutes, during recovery and evacuation for engine repair, for example, you probably won’t have to replace it. Evacuate the AC system for about 30 minutes to boil off any moisture in the lines. Finally, recharge the system with the proper quantities of refrigerant, oil and a little ultraviolet dye for future leak detection.

Check out all the air conditioning system parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on car AC systems, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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