Circuit/System Verification
WARNING: This page is about a different car, the 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
Ignition ON, observe the scan tool fuel tank pressure (FTP) sensor parameter. The reading should be between 1.3-1.7 volts with the fuel cap removed, and change with fuel tank pressure. Increasing pressure causes decreasing voltage, decreasing pressure causes increasing voltage.