Faults are commonplace in large scale systems. These systems experience a variety of faults such as transient, permanent and intermittent. Multi-bit faults are typically not corrected by the hardware resulting in an error. This paper attempts to answer an important question: Given a multi-bit fault in main memory, will it result in an application error -- and hence a recovery algorithm should be invoked -- or can it be safely ignored?We propose an application fault modeling methodology to answer this question. Given a fault signature (a set of attributes comprising of system and application state), we use machine learning to create amodel which predicts whether a multi-bit permanent/transient mainmemory fault will likely result in error. We present the design elements such as the fault injection methodology for covering important data structures, the application and system attributes which should be used for learning the model, the supervised learning algorithms (and potentially ensembles), and important metrics. We use three applications -- NWChem, LULESH and SVM -- as examples for demonstrating the effectiveness of the proposed faultmodeling methodology.