1. The customer interview. Many ethanol-related problems would appear as a cold-starting and driveability complaint immediately following a refueling event.   

2. Identifying a Flex-Fuel Vehicle (FFV). Before September 2006 models, flex-fuel capability couldn’t easily be identified. After that date, the fuel filler lid or gas cap should label the vehicle as a flex-fuel, or the vehicle can be identified as a flex-fuel by a deck lid or fender emblem. In any case, FFV capability is encoded in the vehicle identification number (VIN) and is often indicated during scan tool vehicle identification.   

3. Connect your scan tool. A P0171/174 or P0172/175 DTC indicates a lean or rich condition, which, in addition to other electronic or mechanical failures, might also be caused by an ethanol-related fuel calculation error.   

4. E85 compatibility. Remember that only a minority of vehicles are flex-fuel compatible, so it’s important to consult diagnostic service information regarding a P0171/174 or P0172/175 DTC. Hyundai, for example, issued a TSB in August 2008 indicating no FFV capability existed on 2008 Hyundai models and that fuel system damage can result if E85 is used. The symptoms are generally a loss of power, or a cold-starting or cold-driveability complaint. E85 content can be measured with a Hyundai or aftermarket ethanol test kit.  If the gasoline is ethanol-rich, drain and refill the tank with E10 gasoline.

5. Fuel contamination. The service information for a 2011 Honda Accord with a P0171/174 or P0172 DTC begins with, “Poor fuel quality can also cause DTC P0171/174.” Remember that gasoline can be contaminated with different percentages of diesel fuel. Since diesel fuel doesn’t flow well through gasoline fuel injectors, a “lean” operating condition can result. If no other issues can be identified, drain the fuel tank and refill with fresh E10 gasoline.

6. Hardware failures. A P0172 “rich condition” DTC might indicate a “reset” problem caused by a faulty fuel-level gauge or other hardware failure. Since a fuel gauge failure should set its own DTC, consult your service information to determine the enabling criteria and test procedures for these DTCs.

7. Scan tool bi-directional controls. Be aware that frequent refueling in small quantities can cause the ECM to set rich or lean codes. Under some operating conditions, a scan tool can be used to reset a previously learned fuel trim, so keep an open mind when dealing with flex-fuel problems.