The AC light flashes on the dashboard, and the vent blows warm air instead of cold.
GM has a TSB out for this issue. If the system is full, then the negative battery terminal needs to be disconnected to reset the climate control computer. If the system does not work after this, then gauges will need to be put on, and an AC performance check run, to guide the diagnosis. You may want to enlist the help of a mechanic, such as one from YourMechanic, who has the equipment, training, and certification to inspect and diagnose the issues with your AC system.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing