Conditions for Setting the DTC
WARNING: This page is about a different car, the 2020 Buick Regal and 2019 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
P157A
The ECM detects that the voltage level difference is greater than 11 V between the Wake Up/Enable Voltage and battery supply voltage circuits for greater than 1 s.