Ethanol Fuel causing "Check Engine Light"

Recently customers have visited our shop with a "Check Engine Light" with no performance complaint. When the vehicle was scanned for codes the codes were for a "Lean Exhaust" condition (P0171 & P0174). We checked all the normal lean exhaust code problems; vacuum leaks, low fuel pressure, clogged injectors. All conditions were OK.  It was found that the customer had been buying fuel in small amounts (less than 10 gallons at a time). A check of the fuel showed that the fuel contained over 50% ethanol.  One of these vehicles was a late model truck that was equipped for E85.  Unless the vehicle computer sees at least a 10 gallon change in fuel level, (on an E85-ready vehicle) it will not re-calibrate for alternate fuels, thus causing the "Check Engine Light".  In short, if you are going to use high Ethanol content fuel, you should do so  when you plan to fill up (or at least 1/2 of the tank). 

 It is also worth noting that fuel economy will suffer significantly when using E85 Ethanol vs. Gasoline as is shown in this chart from the E.P.A.