Given that your OC sensitivities apparently end up being zero at least for some grid cells (your plots actually show structure, and even though VERDI by default only shows three significant digits that suggest your min/max values are zero, you can change the scale to see that they are not exactly zero everywhere), commenting out the OM/OC calculation is the right approach. I now see that we actually discussed this very same combine run issue 30 days ago in this thread, and the solution was exactly the same.
None of your second order sensitivities shown in your plots appear reasonable, but this is likely not a combine issue, you probably see unreasonable second order sensitivities for one or more individual PM species in ASENS. combine is computing what it’s being asked to compute, but the very high / infinity values originate from the ASENS files themselves, not combine (in addition to the strange PM25_TOT plot for 2S2 you mentioned, the PM25_TOT plots for 2NX and 2NS show min and max values of infinity, so even though the color scale just shows zeros, the values themselves are not meaningful). So, this isn’t a combine question, but a DDM question that should have its own thread for other people to offer suggestions. In tracking this down, rather than jumping straight to PM25_TOT, I would look at the second order sensitivities (2NX, 2NS, 2S2) for individual species in ASENS or aggregates species like SO4, NO3, etc. in the COMBINE_ASENS file.