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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.