Abstract A high-speed laser absorption technique is employed to resolve spectral transitions of CO $$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO $$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO $$_2$$ 2 asymmetric stretch fundamental bands ( $$\Delta $$ Δ v $$_3$$ 3 = 1) near 4.19 $$\upmu \hbox {m}$$ μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00 $$^0$$ 0 0 and 01 $$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO $$_2$$ 2 non-equilibrium with a single vibrational temperature.