i have a 2003 hummer h2 and i learned the hard way, but for the past year ive been driving my hummer in the summers and i noticed that it gets very hot and i always clicekd the snowflake button. i also noticed that when the snowflake button was off, that the air would be cooler from the vents but i just thought it was random or temporary so i always drove with the snowflake button on. i decided to take it to a local guy on craigslist to get my ac recharged. he seemed very knowledgeable and not scammy, he said that the lines had too much pressure and that it was mostly air, and noticed my ac was short cyclling but then when he got inside he noticed that the snowflake button actually has a line thru it indicating ac off. so he vacuuemed the lines etc and filled with fresh r134a and i was good to do.
i learned the hard way because the ac feels like same temp as it was when i wouldnt click the snowflake button and i hope that r134a in the lines is obviously better then air so whatever.

*SNOWFLAKE BUTTON IS ACTUALLY AC OFF*

he also noticed my condensor was super dirty and noticed how the hummer blocks the condensor so he washed it wiht a pressure washer and got it clean enough. good thing is that i had 0 holes or damage because its hidden back there but it was also very dirty. so make sure you wash your condensor to get all that crud off

 

Lol - yeah GM changed that after 2003 models to eliminate the confusion. There was even a GM tech bulletin for it. Another fun one for new H2 owners is many spend time trying to find the fog lights button as they think the lights on the front bumper are fog lights when they are actually just comically gigantic DRL's.