A frequency-domain approach to balancing of air-fuel ratio (A/F) in a multi-cylinder engine is described. The technique utilizes information from a single Wide-Range Air-Fuel ratio (WRAF) or a single switching (production) O₂ sensor installed in the exhaust manifold of an internal combustion engine to eliminate the imbalances. At the core of the proposed approach is the development of a simple novel method for the characterization of A/F imbalances among the cylinders. The proposed approach provides a direct objective metric for the characterization of the degree of A/F imbalances for diagnostic purposes as well as a methodology for the control of A/F imbalances among various cylinders. The fundamental computational requirement is based on the calculation of a Discrete Fourier Transform (DFT) of the A/F signal as measured by a WRAF or a switching O₂ sensor. For real-time applications, the approach is iterative in nature and the intended goal of A/F balancing is achieved quite accurately and fast in around 1-2 seconds. Experimental results for 3-cylinder and 6-cylinder applications are provided and the advantages and limitations of the technique including the effect of blind spots pointed out. Application of the technique will result in lower tailpipe emissions on FTP and US06 driving cycles as well as tangible (but minor) fuel economy gains. Clearly, in extreme cases of cylinder A/F maldistributions, the impact on the maintenance of good driveability is of paramount importance and achieved automatically using the technique on-board the vehicle in its real-world operation.