Dear Doctor: My 2008 Chevy Impala with 3.9-liter V-6 has a chronic problem with a P0172 code (rich fuel mixture) that my mechanic cannot resolve. The air filter, plugs, O2 sensors, and throttle body have been replaced. The mass air flow, coils, wires, temperature sensor, and fuel pump have been checked and the fuel injectors cleaned. The scanner shows the short-term fuel trim in specification, but the long-term fuel trim is out of specification. There are no misfires, but coil banks 2-5 and 3-6 fail while 1-4 is OK. The engine runs smooth and mileage is good. - Patty
Dear Patty: The one missing link is the information needed to complete the repair without replacement of parts. A quick look on Identifix shows there are multiple P0172 rich codes. The most common repair is an alcohol content relearn or a computer recalibration. (The majority of repairs are the alcohol content relearn). Have your technician subscribe to Identifix for the correct procedure for this rich code repair.