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Discussion Starter #1
So last year I thought my A/C wasn't blowing as cold as before, but as it was a somewhat mild summer, I didn't use the A/C much. A couple of months ago I thought I heard a weird hiss sound from somewhere in the cabin. Today when driving with the defrost on, noticeable hissing from the dash area. Doing a bit of testing, the hiss only occurs when the A/C or defrost is on, so I assume the refrigerant is low.

My plan is to get a bottle of recharge stuff from the parts store (that has dye and oil in it) and recharge. Before that, any obvious failure/leak points I should look for? I did a quick inspection with the car off tracing the A/C lines from the compressor to the condenser and most of the way to the firewall and didn't see any obvious oily residue to suggest a leak. Anywhere else I should look? I'll also plan to do the checks here too.

The car is a 2013 with 95k miles, so I wouldn't be surprised if an o-ring, seal, or hose is leaking slowly.
 

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If you were leaking enough refrigerant to hear a hiss every time you used the AC, it would be empty already.

The hissing is normally coming from the expansion valve as gas passes through the orifice and pressures normalize, or attempt to normalize.

While you may have a leak, and may be low on refrigerant, the noise you hear is highly unlikely to be gas escaping into the atmosphere.
 

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Discussion Starter #4
Thanks. I never suspected the hiss was refrigerant escaping to atmosphere (sorry if I was unclear) but from the pressures not being able to normalize as crjackson wrote.

I checked the fuse, relay, and clutch for proper operation before charging. I added most of a 12oz can of R134a that contains dye (neon green and UV glow) and stop leak (seems like all the refrigerant brands have some amount of stop leak in them). That got the low side pressure to plateau out at 20 psi with the compressor running, which seems reasonable given it's chilly today (55 F). The A/C was blowing quite cold and I didn't want to overcharge, so I stopped.

I traced the refrigerant lines and didn't see any evidence of leaks from them or the compressor. There is one suspicious greasy looking area on the lower part of the condenser where one of the tubes goes into the manifold. There are a lot of fins missing as well (likely from road debris hits). So, we'll see how long the A/C keeps working and whether a new condenser is in my future.

I'll also add a UV blacklight to the shopping list.
 
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