The Magnetic Random Access Memory (MRAM) is considered one of the potential candidates that will replace current on-chip memories (RAM, EEPROM, and flash memory) in the future. The MRAM is fast and does not need a high supply voltage for Read/Write operations. It can also endure almost unlimited Read/Write cycles. These combined advantages of RAM and flash memory make it a potential choice for SOC. In this paper, we present the Write Disturbance Fault (WDF) model for MRAM, i.e., a fault that affects the data stored in the MRAM cells due to excessive magnetic field during the Write operation. The proposed WDF model is justified by chip measurement results. We also construct the SPICE macro model for the magnetic tunneling junction (MTJ) device of the toggle MRAM to obtain circuit simulation results. An MRAM chip has been designed and fabricated using a CMOS-based 0.18μm technology. We also present an MRAM fault simulator called RAMSES-M, based on which we derive the shortest test for the proposed WDF model. The test is shown to be better and more robust as compared with March C-. Finally, we present a March 17N diagnosis algorithm for identifying the WDF.