Diagnostic Aids

WARNING: This page is about a different car, the 2018 GMC Yukon XL, 2018 GMC Yukon, 2018 Chevrolet Tahoe, and 2018 Chevrolet Suburban. However, it is still accessible from the selected car via links, so may be relevant.

The transfer case clockwise circuit shorting to the transfer case counter-clockwise circuit could cause this DTC to set.
It is possible to have windings inside the transfer case motor shorted to the case resulting in a short to ground causing this DTC to set.
The default actions for C0321 may set C0387.
EL-49741 harness is a good source for 9 V.
EL-49742 harness is a good source for B+ and ground.
The transfer case fill and drain plugs are a good source for ground.