I have a 2013 GT that will go from Closed Loop back to Open Loop and stay there after 2 minutes of run time. Car can be idling or cruising down the road. After about 2 minutes the car will go to O/L not ready and various o2 sensor values will go to zero. The A/F display on the dash locks in on Stoich and will not change regardless of actual air fuels. I have verified a .82 to .83 lambda on a NTK wideband under WOT which is running very close to my target after dialing in the maf. The cars o2 sensors will read the same lambda values if you run the car at WOT just after starting it. For some reason the cars o2 sensors go off line around 2 minutes of run time and puts the car back into open loop. The one thing I have not done is flash the car back to stock. Nothing looks out of the ordinary under Open/Closed and Engine Diag Tabs.