I was wondering if it actually possible to see an AC leak by eye. Say one of the aluminum pipes were to leak, would you be able to see a specific type of discoloration caused by the refrigerant around the leaky area?

The reason I am asking that is because I have a leak in my AC (actually had it for a couple of years) and I decided to finally fix it.
I do have a sniffer, but considering the system is completely empty, I don't think I will be able to find the leak that way. I do not want either to inject some refrigerant in as I don't have a recovery tank to purge the sytem afterwards.

Fianally, I thought I could use an ultrasonic detector (which I do not have yet) instead, but I imagine I would also need a compressor (or is a pump just as good?)? Unless the system will anyway build up pressure (when the car + AC are running) even when no refrigerant is present within the system???

  • Welcome to Motor Vehicle Maintenance & Repair! You can get refrigerant with UV dye in it. Commented May 10, 2020 at 14:59
  • Thanks @Pᴀᴜʟsᴛᴇʀ2. I was reading the dye was not very good for your AC system and tried to avoid it. Also, I will still have the problem of not being able to drain my system afterward. Or could I do that by only injecting an insignificant amount of refrigerant, making it ok to release in the atmosphere prior to fixing the leak? Commented May 10, 2020 at 15:08
  • It's about the only way I'm aware of to find a leak visually on an AC system. I don't think a sonic tester is going to be of much use, though I've never tried to use it on an AC system before. Commented May 10, 2020 at 15:10
  • @Pᴀᴜʟsᴛᴇʀ2, Is there any sorts of die I could use that could be injected without refrigerant? Commented May 10, 2020 at 15:12

2 Answers 2


Half the system is built for gas and half the system is built for liquid. In order to effectively find the leak is to have liquid on the liquid side and gas on the gas side. Refrigerant is the only way to complete this condition. The only way to properly do this is to have the proper equipment or hire someone who does.

  • What is the minimum amount of refrigerant I should refil the AC with in order to test the system? Is there any point replacing the accumulator at this point? I guess not, right? Better replacing it after fixing the leak et al? Commented May 12, 2020 at 7:34
  • The amount of refrigerant varies on the vehicle and the cooling capacity. That info can be found in your owners manual.
    – Old_Fossil
    Commented May 14, 2020 at 8:38

An A/C leak will usually also leak some of the compressor oil with it (the oil doesn't just stay in the compressor). The gas evaporates immediately but the oil film stays behind and attracts dust from the engine bay/road.

An oily/dirty A/C pipe near a connector or valve usually indicates leakage.

  • Thanks Matt, I cannot see any oily mark anywhere. I do see some white discoloration on one of the pipes though (at the base of a connector). Looks a bit like what limescale would look like when starting depositing on a metalic surface. It is just benieth the top of the washer tank though, so it might just have been created my the washer product overflow. Commented May 12, 2020 at 7:28

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