2 more things I just thought of. I do have 1 TPM not communicating and 2 while driving the other day I noticed the 4WH Low light flashing but did not engage then a couple days later making a sharp turn in a sand parking lot the system shifted from 4high to 4WH High Lock and it did feel like it engaged. Just throwing it out there to see if it helps to figure out this code P0856

 

Dead end?

 

2 additional ideas; The 4wd switches on the dash could have crud inside them. Remove, and use a good contact cleaner (CRC Quick Dry Electronic Cleaner is a good one), along with compressed air. Another possibility is the transfer case encoder motor; expensive but easy to install and a fairly common cause of 4wd mode issues on many GM vehicles.

 

thank you

 

Any update?

 

Finally gave up and sold the H3t. Loved the truck just not all the issues they are known for. Fixed most of them but it was time to move on, sad but true