2018 Sierra 8 speed, only have 4th gear, delay shift into D

[Bmech]

This morning I started my truck to go run some errands and the check engine light came on before I pulled out of the driveway.  When I backed out and put it in D, nothing happened.  Tried again and no go.  Barely revved the engine and then finally went in gear.  Drove down the road and never felt a shift.  Was going around 30-35 mph at 2500-2800 rpm. Sped up and slowed down, never shifted.  I am guessing i only have 4th gear.  Took straight to the dealership.  Tech looked at today but couldn't figure it out.  They said they should know something next week.  Truck only has 12,000 miles...  Hopefully its an electrical issue, we shall find out next week.  Anyone else have a similar issue?

Edited December 12, 2020 by Bmech
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[Lsgun1]

Sounds like limp mode. Could happen for a bunch of reasons.

[Bmech]

Dealership still has the truck.  Now they say its not acting up.  At least the svc advisor got all the active codes when I brought the truck in.  Transmission codes, loss of communication, etc.