There are certain things an N/I dealer simply can't do for you because of legal considerations, and they have to go "by the book" for airbag diagnosis, period. Nissan doesn't actually publish diagnosis info, the DTC's in the SRS section will basically just say, "replace A, replace B, replace C". That's lawyers, but they're stuck with that.
Here's what you can do yourself to diagnose and maybe repair the issue (which is very likely wiring). WARNING: DISCONNECT THE BATTERY NEGATIVE TERMINAL BEFORE PERFORMING ANY OF THIS:
1) Disconnect the suspect airbag connector and bridge the connector contacts with a 2 ohm, 1/4 watt resistor. Place some tape over it so you can drive around that way if necessary. Reconnect the battery and:
a) if the DTC was "current", scan the ACU and see if it will erase. If so, the bag is bad.
b) if the DTC was "past", erase it and drive around for awhile to see if it stays erased. If it doesn't recur, the bag is bad.
2) If the bag is vindicated, access the ACU and bag connectors and check the pin-fits using a safety pin of 0.024~0.026" diameter. The pin should palpably "drag" on the spring contacts when inserted. If not, replace the bad pin or the entire pigtail.
3) If pin-fits are good, ohm the wires between the ACU and bag. Continuity should be excellent end-to-end, nothing above 0.5 ohm is acceptable. With both ends disconnected, you should also see infinity:
a) between the two wires
b) between each wire and 12V power
c) between each wire and ground
4) If you don't find any obvious issues, then you probably have a partial break somewhere in the wiring between the bag and ACU that's separating when the car is driven. The best solution is simply to wire around the problem by clipping the old wires at both ends and routing brand new ones on the outside of the harness. Use "solder shrink" connectors of the right size, don't use regular butt connectors and don't use hand-soldering unless you're very good at it. Solder "gloms" can have enough innate capacitance to cause additional problems.
Hope this helps you.