A bad ground connection of KIA (from TIEPIE) http://www.tiepie.com/en/automotive/articles/KiaCarensMixtureIssue
A Kia Carens with 1.6 liter engine keeps generating an oxygen sensor (or lambda sensor) related code that the fuel mixture was too rich. Resetting the error and driving the car would set the error code again.
Due to the voltage drop in the ground connection, the MAF sensor generated a higher output signal. A higher MAF sensor signal means more air entering the cylinder, requiring more fuel for optimal combustion. In this case, the MAF sensor signal is too high, causing the ECM to think that more air enters the cylinder than actually does, which results in too much fuel to be injected.