When you hear a hissing sound from your car air conditioner, it's important to understand what it could mean and how to proceed. The hissing noise could be caused by several issues, from simple to complex, that can lead to your AC not working properly or even making no noise at all. Some common hissing sounds from your car ac include hissing from the compressor, hissing from hoses or fittings, hissing from the expansion valve, hissing from the condenser and more.
The first and most common reason for hissing from your car ac is an internal leak of the refrigerant gas. There are two sides of the AC system, a high pressure side and a low pressure side. The refrigerant (called Freon) moves between these two separate sides to regulate pressure, and small holes or breakage in any of the seals can cause the hissing noise.
A hissing noise from your AC system when you turn it off is actually a normal, harmless process that's done to equalize pressure in the system. The internal valves are designed to keep the freon pressurized, and when you shut off your air conditioning it causes the hissing sound for a few seconds while the system is still transferring the refrigerant from the low to high pressure side of the AC.
If you're hearing a hissing sound from your car ac that persists after turning off the AC, it may be time to replace the evaporator coil. The evaporator coil is responsible for cooling the refrigerant as it moves from the compressor to the hoses and vents, and damage or wear can cause it to start to deteriorate or break down.