[garydoe]

Driving at 60-70 mph and in hold mode, I experienced a slight loss of power, twice. Later I noticed the engine light lit. OnStar reported p0700 and p0756 errors. I took my 2013 to dealer and they said I would need to bring it back the following Monday for more extensive diagnostics and that they would give me a loaner then. The next morning I noticed the engine light was off (don't remember looking checking after leaving the dealer). Called OnStar and they could detect no codes now.

I suspect it's the solenoid, from the codes.
Can engine light be reset and codes turned off too, by the dealer.
I plan to test the hold mode tomorrow on the highway.

 

[Upper5Percent]

p0700 - Transmission Control Module(TCM) Requested
Transmission Emissions-Related DTC set
MIL Illum is for Type A, 1 trip

p0756 - Shift Solenoid Valve B Stuck Off
This DTC will indicate when Shift Solenoid Valve B (Y Valve)is stuck in the hydraulically low position
The Y valve is determined to be in a hydraulically Low state when it has been commanded hydraulically High.
This detection only occurs during an Y valve transition
MIL Illum is for One Trip, Type A

Both of these codes are 1 Trip

 

[mpmoore1979]

I had a 2013 in for the same issue, same codes. The problem was in the valve body of the transmission/drive unit. The repair was covered under the Voltec warranty so if you are still under this coverage then it should be repaired at no charge. If the valve was stuck and then freed up, the engine light will go out but the codes will still be stored for a while. I would have them check out the car, if the problem comes back when you are out of warranty then you are out of pocket.