Fuel tanks

I believe like all Gm vehicles, they will run with the EPA limits. E-10, 10% Ethanol.
My wife's Envoy, had some O2 Sensor problems and the fuel in her tank was measuring 16%. Dealer told me that Gas may be the issue. I asked them to Re-program the Computer with more of a Variable limit.

The answer was, it can't be done. Mixture is calculated by the Engineers at the design level of the motor, and are calculated based on what the EPA sets as the Nationwide standard. A curve is built into the Equation but cannot be altered unless the designers recalculate and the update is approved by the EPA. It all has to do with the Mixture, and how the engine burns it.
Emmisions Inspections.
???
Sounds like a good answer!