 Once you hit the highway speeds, roll up the windows, then switch to circulating mode. Check and replace the cabin air filter every 15,000 miles or as needed, and the filters are sold at just about every auto parts store and can be quickly installed as a DIY project. If you suspect your AC is not working properly, have a trusted mechanic check it for leaks or replace refrigerant as needed. AAA Idaho Public Affairs Director Matthew Conde said, make sure that you and everyone in your group stays hydrated and maintains a safe body temperature.