When we were camping in the San juans above 12000 ft. I just couldn't sleep as I kept gasping for air, so I got up to take a shower about 4AM (in September) and heard the water heater cycle on when I was in the shower. I was finishing up having washed everything and i heard THE LOUDEST NOISE i'd ever heard: a squeal-a schreek. It was the CO2 alarm. The book said to vacate the premisis if the thing goes of and call the fire department. None found. Meanwhile I was standing outside with just a down jacket on waiting for the squeal to stop.
We figured it was the water heater running way too rich on propane at that alt. that tripped the alarm.
Even my Honda 2K generator has 'jet kits' for different elevations:
0-4000 = stock jets
4K-6K feet = jets
7K-9K feet = high altitude jets.
I've never changed them even though I do have the whole set since the red box seems to muddle on no matter what.
jefe