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Car AC Lines Freezing Up? Here’s What You Can Do About It

An air vent in a car dashboard.

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 Outside?

If you open your hood and find that one or more AC lines look like a popsicle, you may have a problem with your AC system. More than likely the amount of refrigerant in the system is low. Your AC system operates at peak performance in a narrow area that requires the right amount of system pressure and volume of refrigerant. An imbalance can cause issues such as an overworked compressor or components that don’t work like they are designed. Iced up AC lines could also mean a problem with the low pressure switch, which prevents the AC compressor from cycling on and off. A not so common problem could be a dirty or clogged AC condenser. The AC evaporator is under the dashboard and is what does the actual cooling of the air inside the passenger compartment. Without enough air flow to cause proper heat transfer the condenser itself and the lines attached to it can become extremely cold, even to the point of frosting over from moisture in the air. A similar condition can happen if the cabin air filter is extremely clogged.

What Causes Car AC Lines to Freeze Up Inside?

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.

Car AC lines freezing up? Here

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 Inside 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.

What Can You Do About Moisture Outside the System?

Moisture (and the ice it forms) will go away on its own once the AC system is working correctly. Moisture is not leaking out of the AC system, the moisture is only a symptom. Fix the AC system and the moisture will solve itself. The ice will melt naturally in the ambient air, no need to help it along.

Check out all the air conditioning system parts available on NAPAonline or trust one of our 17,000 NAPA Auto Care 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.

Photo courtesy of Pic Basement.


Benjamin Jerew View All

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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