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Phantom Stink

Drive around making multiple stops; and sometime at random times the a/c with stink terribly but will dissipate in a couple of minutes. Makes no difference if the recirculation is active or not. Classic example, I drive 5 miles to my mechanic and no problem. I start driving home, get 1 mile and am overwhelmed with stink. Turn-around and by the time the mechanic sticks his head in the car; no problem.
Mechanic has probed around with a peep scope inside and out and finds nothing in the air duct system. Drain hole is just fine. Had GMC update the firmware to keep the heater on after the car is turned off; no difference.
Have tried industrial Fabreez-like and Ultrashield odor treatments to no effect.

Do you have a cabin air filter in your car? most new cars have one hidden behind the glove box and need replacing when clogged.

The cabin air filter was the first thing checked and now has been replace three times even though they were in good shape

Any skunks live near where you drive?

Are you certain it’s inside your car, and not outside? Driving past a cabbage patch after a harvest can be a life long memory.

Does it happen when you are alone, or with others? Do they complain, too?

What does it smell like? Skunk, sulfur, rotting eggs, vomit, etc.

I was on the same page as @wentwest!

I went through Wisconsin Rapids three times this past weekend and had no problem. Sunday I went through and the wind was just right. There was a terrible smell from the Paper Mill as the wind was just right.

Where I live there is a Carnation Foods plant. Most days are fine but when they are packaging Instant Coco Mix you can tell. While it’s not an offensive smell, it sure is noticeable.

Yosemite.

Very rare to find skunks but we do have problem rats. This has been going on for three years and it happens continually but randomly.

This is southern California where it is warm and dry. Also, no industry or farming that could be the issue.
It smells like a cross between dead rat and sweaty arm pits. The smell is never in the car by itself.

The smell occurs everywhere but just at odd times while driving. It will come and go on the same trip and then not be around for a while, then you will start up from a stop (already stopped many times) and stinkeroo; air out the car and be gone in a few minutes.

Several decades ago, I visited the area of Ft. Ticonderoga with my parents, and we really wanted to view the Revolutionary War sites in the area. However, the stench in the area drove us away. A local informed us that the stench was from the local paper mill.

Make sure the ac is draining, not sure about your gm, but after a significant rain, I heard sloshing in the driver door of my 2003 trailblazer, It was full of water! Not knowing any better and caring little about long term effects, I drilled a hole in the bottom of the interior part of the door and let the water drain out. Then I noticed there was a lower spot, and said I should put a hole there, lo and behold there was a hole there, but it was clogged with debris, cleaned it out and the rest of the water drained out.

Moral of the story you may have plugged drain holes in your door that are growing mold and mildew or stinky smells etc.

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Could be fungus growing on the ac evaporator core

Are there special products available for that problem? If not I’d be inclined to try an experiment, a spray bottle filled with a dilute mixture of bleach and try to get the AC system to suck in a fine spray from the air vents under the windshield.

Very corrosive to aluminum…bye bye evaporator…

;-]

Yes

There should be plenty in the typical auto parts store

Yup!
Frigi-fresh is readily available, and it works very well.

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The root problem has been discovered after 3 years of messing with this car. Mice are living in the frame of the car (not a sealed system) and move into the AC inlet which is connected (mouse sized) to do their potties. Nothing is dead but they retreated with my special mouse-be-gone chemical (ammonia).

I smell a prank. Have you pranked anyone in the last few years. Does someone owe you revenge?

Good for you for getting to the bottom of the problem. Best of luck and Happy motoring.