The HFI validation is mostly modular. That is, part of the pipeline, be it timeline processing, map-making, or other, validates the results of its work.
Top-down approach to systematics
Yardstick simulations (including a Sisyphe summary of what can be neglected) (Deluis/Montier)
Simulations versus data results (including PTE) (Techene)
(Planck) High Frequency Instrument