I cant give you an answer regards your car but i have had previous with seatbelt/pretensioner failure.These units are not serviceable and contain a small explosive device which fires a pin locking belt in a crash,therefore you wouldn't want to fiddle with it anyway.You can check the electrical cable that connects to the mechanism hasn't worked loose...highly unlikely unless youve had pillar trim off and dislodged it yourself.
More probable its faulty which will either require and complete belt/PT replacement or specialist who may be able test and fix.
I had to change the srs module to clear the warning light....even though i had replaced the seatbelt/PT the car would not recognise it until i had error code removed.Any problem logged with airbag/seatbelt cant easily be cleared,figures given the importance of it.I just found it easier and much cheaper to source a replacement module.Remember this was not on a honda but im quite confident this will apply to most vehicles.
It's sounding like it could be in the loom then, but check the connector block carefully first.
Also run a meter on the two units you now have to see if the resistance vales are the same on each (you might have a second faulty unit of course but if the second is a new one it suggests the issue isn't with the tensioner)
If no go try disconnecting the tensioner unit completely and running diagnostics again to see if you get a different error code. before attempting to connect directly back at the fusebox.