Spring is over, and it's time to start using the a/c while I drive, but when I tried to use it for the first time this year, it wasn't cold. Room temperature at best. I decided to try recharging it myself with A/C Pro.

I checked to make sure the compressor was turning. It was. I turned my a/c all the way to maximum, and started to recharge. The pressure was super low according to the a/c pro gauge. After a while it was getting into the green area, but then the pressure started dropping again. Eventually the can of freon was empty, so I stopped. The compressor was still fine, no funny business suggesting that I overcharged the a/c. I checked the air blowing inside my car, and it was definitely colder, but not really cold like it used to be :-(

When I turned my car off to start cleaning up, I noticed I could hear a hissing sound from under the hood. I turned my car back on and noticed I could hear it a bit in the car as well.

I guess I'll know tomorrow if the freon leaks out and my a/c doesn't work again, but in the mean time, could there be a problem here? I considered using Super Seal and then putting in more freon. Would this be a bad idea? I'd really like to avoid a trip to the mechanic.

  • Welcome to Motor Vehicle Maintenance & Repair! May 26, 2018 at 23:24

3 Answers 3


Try looking at the Schrader valve in the fitting. If the hissing is coming from there, quickly and lightly press on the valve to help it seat. This is a common issue especially on older vehicles when debris gets caught in the valve. Check under the hood for the black sticker that shows refrigerant specs. Compare that to the amount added from the can. Some only take a few ounces while some take 2 or more pounds. When it comes to sealant, don’t use it because it clogs AC recovery machines and no shop will want to work on your system. You might want to consider a vacuum pump and a proper set of AC gauges. The pump will help diagnose whether you have a leak or not, and will ensure all water and air are removed from the system. AC compressors are not designed to handle moisture and will fail. Run the pump for about 10 minutes, close the service valve and wait another 15. If your gauges continue to read 30in Hg, you don’t have a leak, and are ready to recharge.


The hissing sound is most likely the refrigerant going through the expansion valve. It's how the AC works. The expansion valve allows the cooled and liquified refrigerant to expand, becoming really cold, which then flows through the evaporator. The air inside the cabin flows through the evaporator, which then cools the air. If the A/C were leaking bad enough for you to hear a hiss, you wouldn't even be getting any cool air out of the system (before or after). The only way there could be an issue is if you overcharged the system and blew out a seal.

One thing here ... if you used a full can of AC Pro, you most likely do have a leak in your system. You'll probably want to take your vehicle to an A/C shop to have it fixed. You can sometimes get away with topping off your system, but an entire can of AC Pro is quite a bit of refrigerant.


Tap the valve the valve stem is loose. You can tap it back in place before you lose the rest of your coolant. Probably have to refill it again

  • 1
    Hi Amber - welcome. Please read existing answers before posting. This was already covered in an answer.
    – Rory Alsop
    Sep 3, 2022 at 11:41

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