i havent even looked if my micra has a belt tensioner , but on 99% of cars i have had a seatbelt issue, its been a dirty/damp/disconnected/damaged plug under the seat.
usually damp shoes, kids toys jammed when the seats moved.
every technician jumps on the . diagnostics except me. in 30yrs iv found 2 with a fault. both merc's.
take a small hand mirror and inspect wires both sides, wires can get nipped on the side you cant see.