Conditions for Setting the DTC
WARNING: This page is about a different car, the 2011 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
P0851
The Park/Neutral switch circuit voltage is low, 0 volts, for 0.01 seconds while the TCM is reporting the gear selector is in a reverse or drive gear.
P0852
The Park/Neutral switch circuit is high, 12 volts, for 0.01 seconds while the TCM is reporting the gear selector is in Park or Neutral.