To address this challenging task, we propose innovative SoX diagnostics method based on a computationally efficient multi-scale simulation framework for PEMFCs, parameter identification techniques and methods for assessing uniqueness of parameter identification. Proposed state-of-the-art simulation framework incorporates advanced submodel on spatially resolved dynamics of liquid water and newly added submodel considering the impact of the gas crossover effects on mixed potentials. Results demonstrate capability of the innovative SoX diagnostics method to perform detailed diagnostics of internal states of the PEMFC and to pinpoint degradation-imposed variation of parameters which are interlinked with catalyst activity, membrane losses and gas diffusion layer losses.