Conditions for Setting the DTC
WARNING: This page is about a different car, the 2019 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
P305B
The control module detects greater than 6 V or greater on the ignition voltage circuit, with the ignition turned OFF.
P305C
The control module detects 6 V or less on the ignition voltage circuit, with the ignition turned ON.