Hissing Noise from Car AC (Causes and How to Fix it)

Air conditioning in a car is commonly one of the first things to give owners major headaches. Car ACs have a wide range of costs and severity of problems. A common symptom warning that something is wrong with the AC is a hissing sound.

The annoying part is that a hissing sound from the car AC could be a legitimate problem or normal air conditioning noise.

Why is Your Car AC Making a Hissing Sound?

There are five potential reasons your car AC is hissing.

Refrigerant Gas in Expansion Valve

If the hissing occurs for a few moments while the car is starting, it could just be the refrigerant gas. This gas is what makes the air cold. The gas moves through the unit and valves. It can cause a hissing noise but usually quiets down.

Refrigerant Leaks

Refrigerant leaks are a legitimate problem for your car AC. These leaks can happen at any part of the air conditioning process. Small holes or broken seals happen for several reasons. Wear and tear and physical damage cause parts of your unit to break down. Essentially, gas is escaping somewhere within your unit.

Bad Condenser Fan Motor

A broken or defective condenser fan will result in a pressure buildup in the AC compressor. There could be debris blocking the airflow for the fan. Without airflow, the AC won’t blow cold air. The hissing could be from the blocked airflow trying to squeak out.

Overcharged System

An overcharged system in the car AC means there is too much refrigerant or oil. The excess refrigerant or oil is another way the compressor becomes overwhelmed with pressure. The hissing is the excess gas trying to escape.

Internal Valve Leak

Internal valves in your car AC unit keep the freon pressurized. If there is a leak, there is no cold air. The internal valves are identical to the ones on car tires. Internal valve leaks are common and easily fixed. The leaking pressure will make a hissing sound.

Car Air Conditioning Hissing When Off

Sometimes when you turn the car off, there is a hissing sound. Luckily, this sound is pretty much harmless. The hissing sound from the car AC when the car is off is the car equalizing pressure.

The air conditioning unit has a low-pressure side and a high-pressure side. The refrigerant moves from each side through tubes and valves. When the car is turned off, the refrigerant needs to equalize. The hissing noise is just the sound of excess gas from one side moving to the other.

What About Loud Fan Noise when AC is on in the Car?

Loud fan noise when the car AC is running is most likely a bad blower motor. The blower motor is what turns the fan to blow the air. A bad motor may be working extra hard to blow air because of debris. If the sounds get louder as you crank the AC, you probably have an issue with the blower motor.

The blower motor can be diagnosed and even fixed without a mechanic.

  1. Turn the AC up to see if the noise gets louder
  2. If it does, find the blower motor under the glove box
  3. Check the motor for debris and remove anything you find
  4. Reinsert and test if the sound continues

How to Fix Hissing Sound in Car AC

The method of fixing the hissing sound will depend on the severity of the problem. Some of these issues need to be handled by a mechanic.

AC Superseal

An AC super seal is a DIY solution for refrigerant leaks. It is an inexpensive option if you have diagnosed it as a leak.

The sealer goes in as a liquid but hardens when exposed to the temperature of the leak. This seals the leak and allows the refrigerant to flow normally.

Air conditioning sealant also has the added effect of repairing metal AC components and flushing out excess moisture living in the system.

Replace the Leaky Valve

If the Schrader Valve is leaking, replacing it is a cheap option. It is the only AC valve that you can replace without a professional. Schrader valves are identical to tire valves. The other type of valve is the ball valve which controls the high-pressure side of the AC. Only a mechanic should handle ball valves.

You can screw off the Shrader valve to determine if it is the culprit. There may be freon bubbles or excess oil in the cap. This indicates that the valve needs to be changed.

Replace the Condenser Fan

A bad condenser fan would be the cause of high-pressure buildup in the AC condenser. It is possible to change a busted condenser fan at home. It should only be done if you can guarantee that the condenser fan is broken.

If you check your operator’s manual, you can decipher which is the AC condenser fan underneath the hood. If you run the engine and it is not running then it needs to be replaced.

Go to a Mechanic

Seeking a professional to help fix the hissing sound in the car AC is sometimes all you can do. Overcharged systems need to be handled by professionals exclusively. You can measure the pressure of your system at home, but only a mechanic can release pressure.

Anytime you are unsure about your car repairability, seek professional help. Mechanics can always diagnose and recommend repairs.

How Much Does it Cost to Fix the Hissing Noise in Car AC?

The cost of repairs will depend on which component of the AC is malfunctioning. Going to a mechanic will cost at least $100 just to diagnose. Minor repairs will cost up to $800. Major repairs will be a few thousand dollars.

Sometimes you can repair the problem at home. AC super seal and Schrader valves are cheap. Both can be purchased at a local hardware store.

Compressor problems are always the most expensive repairs. Sometimes it is just best to replace the AC entirely.

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Author: DJ

Dave Junior is a hands-on automotive technician with experience in performing service, diagnostics, and repairs on domestic and imported vehicles. He enjoys writing and sharing his knowledge far and wide.

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