Ethanol fuel

Had a 2005 impala in shop,ran like crap. Replaced spark plugs and reset ethanol sensor. Code was for ethanol sensor out of range.after reset it showed 19% ethanol. Instructed customer to go fill up with premium and i gave him sea foam to add. Left my yard, check engine light came back on in less than a mile. Filled car up and within three days the symptoms cleared and never returned. He switched fuel stations after that. Guess what,his fuel mileage improved. There is no gain with ethanol,only loss,right out of the consumer’s pockets.