Diagnostic Aids
WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.
The HVAC system may not be able to properly control or maintain the selected mode if the vacuum supplied to the HVAC control assembly is insufficient.
Inspect the vacuum supply hoses for kinks, leaks and proper connections, and inspect the vacuum tanks tank for leaks if the system is unable to switch to or maintain the selected mode.