Check Engine Light - Fuel Too Lean

My wife put E85 fuel in my 2015 Cadallic Escalade and of course the check
engine light came on.  Took it into the dealer and codes read too lean of fuel.  
Recommended to drain tank at the cost of $800, so decided to do it myself.  I
was able to drain about 3/4 of the tank and refilled with 87 octane.  The
check engine light is still on and I'm trying to clear the codes with my own
hand held scanner.  However, the codes will not clear.  Do you have to got
back to the dealer and have them clear the code or will it eventually go back
off once the fuel mixture is correct.  Thank you!