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This is the second paper of a series making use of Herschel/PACS spectroscopyof evolved stars in the THROES catalogue to study the inner regions of theircircumstellar envelopes (CSEs). We analyze the CO emission spectra, including alarge number of high-$J$ CO lines (from $J$=14-13 to $J$=45-44), as a proxy forthe warm molecular gas in the CSEs of a sample of bright carbon-rich starsspanning different evolutionary stages from the Asymptotic Giant Branch (AGB)to the young planetary nebulae (PNe) phase. We use the rotational diagram (RD)to derive rotational temperatures ($T_{\rm rot}$) and masses ($M_{\rm H_2}$) ofthe envelope layers where the CO transitions arise. We also obtain a firstorder estimate of the mass-loss rates and assess the impact of the opacitycorrection for a range of characteristic envelope radii. We use multi-epochspectra for the well studied C-rich envelope IRC+10216 to investigate theimpact of CO flux variability on the values of $T_{\rm rot}$ and $M_{\rm H_2}$.PACS sensitivity allowed the study of higher rotational numbers than before,indicating the presence of a significant amount of warmer gas ($\sim$200-900 K)not traceable with lower-$J$ CO observations at sub-mm/mm wavelengths. Themasses are in the range $\sim10^{-2}-10^{-5}\,\rm M_{\odot}$, anti-correlatedwith temperature. For some strong CO emitters we infer a double temperature(warm $T_{\rm rot}\sim$400 K and hot $T_{\rm rot}\sim$820 K) component. Fromthe analysis of IRC+10216, we corroborate that the effect of line variabilityis perceptible on the $T_{\rm rot}$ of the hot component only, and certainlyinsignificant on $M_{\rm H_2}$ and, hence, the mass-loss rate. Therefore, theparameters derived from the RD are robust even when strong line fluxvariability occurs, with the major source of uncertainty in the estimate of themass-loss rate being the size of the CO-emitting volume.

Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes