Kept getting the above DTC P0D59 latching on my Gen1. Didn't affect charging and so I would just clear it via the OBD-II tool but it would then just latch again after a couple of days.
Funny thing about this DTC or the corresponding DTC P0D58: Proximity Detection Circuit Low Voltage are only checked for when NOT plugged in and charging so this code would latch randomly while I was out driving.
What resolved this issue on my Gen1 was the replacement of the X98 hybrid battery charger receptacle which as evidenced by the pic above had an obvious crack in the waterproof housing on the rear of the receptacle that wasn't visible until I removed it!
BTW: Even though this would have been normally covered by the 8yr Voltec warranty, I was able to get the replacement X98 receptacle GM part#22822889 fairly cheap and decided to just perform the work to save myself a trip to the GM service dept.
It was a simple install and I only needed a T-30 in order to remove the 4 torx screws that secures the housing!