Natural gas is one of the most attractive alternate to gasoline and diesel fuels due to its competitive price resulting from its abundant availability and lower GHG emissions. In stoichiometric natural gas engines, three-way catalysts (TWCs) are widely employed to convert the exhaust pollutants CO, HC (including CH4) and NOx to CO2, N2 and H2O. TWCs contain Platinum Group Metals (PGM) as the active centers for various reactions and ceria-zirconia (CeZrOx) material to provide oxygen storage capacity (OSC), which acts as a buffer for maintaining stoichiometric conditions under a broad range of air to fuel ratios. The activities of the PGM and OSC components of TWC degrade upon prolonged real life operation. This degradation could arise due to various factors such as high temperature hydrothermal exposure of catalysts for prolonged periods and masking/poisoning of the catalytic sites by, for example, chemical contaminates originating from lube oil consumption and/or coolant leakage. Isolating the impact of individual degradation mechanisms on TWC performance is imperative for developing improved catalysts, controls and performance recovery methodologies. In this work, we developed performance evaluation methodologies to quantify the correlation between NOx conversion and the amount of OSC in TWCs. By using this relationship combined with the catalyst characterizations such as BET, EDX and ICP, we established a laboratory protocol that is able to distinguish the impact of different aging mechanisms. Throughout the lab diagnostics on high milage field-aged TWC systems, it is found that chemical contaminants such as P, S, Na and K containing species were concentrated on the inlet of the real-world-aged TWCs. Lower NOx conversion was observed on the chemical contaminated locations, however, the performance of the rest of the catalyst unit was barely impacted due to its sharp axial gradient. Compared with chemical contamination, hydrothermal aging has a more pronounced impact on the system level performance and therefore is the key aging mechanism for the field-aged TWCs.