This paper performed a comprehensive study of the thermal nonequilibrium effects of CO/Ar mixtures with various degrees of N-2 additions and probed the N-2 relaxation behaviors via the CO rovibrational thermometry. The rovibrational temperature time histories of shock-heated CO/N-2/Ar mixtures were measured via a laser-absorption technique, and the corresponding vibrational relaxation data were summarized at 1890-3490 K. The measured results were compared with predictions from the Schwartz-Slawsky-Herzfeld (SSH) formula and the state-to-state (StS) approach (treating CO and N-2 as pseudo-species). The vibrational state-specific inelastic rate coefficients for N-2-N-2 collisions were supplemented using the mixed quantum-classical calculations. The StS predictions, informed by experimentally measured pressures, showed good agreement with experimental data. Additionally, the impact of coupling between flow dynamics and StS kinetics behind reflected shock waves was evaluated using two different one-dimensional approaches, which provide limiting bounds (accounting for unsteady flow and end wall effects) in post-reflected shock flow conditions. Moreover, the vibrational relaxation data of the N-2-N-2 system were modified via sensitivity analysis to improve the performance of the SSH formula. Further analysis highlighted that the vibration-vibration-translation path provides an efficient way for vibrational energy transfer between CO and N-2, resulting in almost the same vibrational temperature time histories for CO and N-2. Therefore, the N-2 relaxation behaviors can be characterized by the CO rovibrational thermometry, considering N-2 is infrared inactive. Finally, the heat sink effects and the reflected-shock-bifurcation phenomena were highlighted.