Single-sided linear induction motors (SLIMs) are used in maglev levitation systems for propelling purpose. The reasons for such usages are their capability in direct drive applications with very good dynamic performance, high reliability and high flexibility in trajectory generation. High-performance requirements of the machine demand a high saliency ratio resulting in small air gap length along the direct axis. Therefore, the air gap symmetry in the primary side and consequently the air gap asymmetry fault diagnosis is essential for the system maintenance. In this study, the air gap asymmetry fault in a SLIM is diagnosed. Since the precise definition of the air gap asymmetry fault and fault severity has not been so far introduced in the relevant literature, they are given here. The proposed fault index is the amplitude of the sideband components of the current with a particular frequency pattern. First, mathematical analysis of magnetic field is proposed, then, fast Fourier transform is applied to the current waveform in the SLIM. The spectrum of the SLIM current in the healthy and faulty cases under different loads is obtained. Time-stepping finite element method is applied to simulate the healthy and faulty motor. Experimental results are verified by the introduced theoretical index.