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I'd say that you may have an issue with the high power fuses or the main battery connections or the main earth connections
so I'd begin by checking that the main battery cables are on tight and the terminals are clean and bright. Do the same for the engine / gearbox earth strap.Then follow the main positive lead back to the engine and down to the starter motor, and check the connection at this end as well.
then check the high power ( 30A+) fuses for connection corrosion and damaged cracked fuses
the HDC error can pop up if you have low battery voltage or bad connection
or it could actually be a fault with the hill descent