In this joint experimental-theoretical work, we utilize attosecond FWM spectroscopy to investigate the ultrafast decay dynamics of autoionizing Rydberg states of CO2. Spatially isolated FWM emission signals are primarily observed at energies associated with members of the Henning sharp series between 17.3 -18.1 eV. However, transient measurements, supported by theoretical calculations of autoionization lifetimes, reveal that these emission signals report on the dynamics of members of the shorter-lived Henning diffuse series instead. Specifically, the experimentally measured decays of 33 ± 6 fs, 53 ± 2 fs, and 94 ± 2 fs correspond well to the lifetimes of the n = 6 -8 Henning diffuse states calculated here. The interpretation of this counterintuitive result is facilitated by calculating oscillator strengths for transitions between states that may participate in the various resonant wave-mixing schemes. These calculations indicate that, due to substantial ground to diffuse state and dark to sharp state oscillator strengths, wave-mixing pathways dominate that probe the Henning diffuse states but emit from members of the Henning sharp series, as observed experimentally. This attosecond XUV FWM experiment demonstrates that transient wave-mixing can be successfully utilized to monitor ultrafast decay in polyatomic molecules with high selectivity.