diff --git a/src/common/m_mpi_common.fpp b/src/common/m_mpi_common.fpp
index da5f07b0a7..3f6e34f6c5 100644
--- a/src/common/m_mpi_common.fpp
+++ b/src/common/m_mpi_common.fpp
@@ -571,7 +571,14 @@ contains
             v_size = nVar + 2*nb*nnode
             buffer_counts = (/buff_size*v_size*(n + 1)*(p + 1), buff_size*v_size*(m + 2*buff_size + 1)*(p + 1), &
                              & buff_size*v_size*(m + 2*buff_size + 1)*(n + 2*buff_size + 1)/)
+#ifdef MFC_SIMULATION
         else if (present(q_T_sf) .and. chemistry .and. chem_params%diffusion) then
+#else
+        else if (present(q_T_sf) .and. chemistry) then
+            ! post_process converts cons->prim over the ghost-inclusive bounds, so the temperature
+            ! Newton guess must be valid at rank seams for EVERY chemistry run (not only diffusion):
+            ! an unexchanged seam ghost is an uninitialized guess -> NaN T/pres/c in the output
+#endif
             chem_diff_comm = .true.
             v_size = nVar + 1
             buffer_counts = (/buff_size*v_size*(n + 1)*(p + 1), buff_size*v_size*(m + 2*buff_size + 1)*(p + 1), &
diff --git a/src/post_process/m_start_up.fpp b/src/post_process/m_start_up.fpp
index 9b9c9d876e..3b294ff075 100644
--- a/src/post_process/m_start_up.fpp
+++ b/src/post_process/m_start_up.fpp
@@ -155,7 +155,11 @@ contains
 
         call s_read_data_files(t_step)
 
-        if (chemistry) call s_compute_q_T_sf(q_T_sf, q_cons_vf, idwbuff)
+        ! seed the chemistry temperature over the INTERIOR only (mirrors the simulation,
+        ! m_start_up): the ghost q_cons is unread at this point, so a ghost-inclusive sweep
+        ! would Newton-iterate on garbage (NaN under NaN-init builds) at rank seams and
+        ! physical boundaries; s_populate_variables_buffers below extends q_T into the ghosts
+        if (chemistry) call s_compute_q_T_sf(q_T_sf, q_cons_vf, idwint)
 
         if (buff_size > 0) then
             call s_populate_grid_variables_buffers()