While driving our 2000 Nissan Xterra, within a matter of minutes the A/C stopped working, the air bag light came on, the seatbelt light came on, and the radio turned off. Later in the day, it wouldn't start, possibly a dead battery, but you could hear the starter clicking.

If you have a heavy enough object sitting in the passenger seat, the sensor may think a person is sitting there without a seatbelt on....or in a case that I ran into a few years back, a small coin had lodged inside my locking side seat belt. The side with the release button. Or lastly, have you vacuumed your car recently? You may have unplugged your seatbelt sensor wires while vacuuming.
Your car is getting up in age too, so check to see if the retracting spring is still pulling the seatbelt back into its holster, check the general condition of your locks on the belts....not sure if there is a fuse link for a seatbelt system in your car...but you could check your owners manual to see if there is, and while in the manual...check for reasons the seatbelt light comes on too. You can get a copy of your owners manual on line from Nissan or a aftermarket source. Good luck and sorry I couldn't be more specific. Your owners manual or an on line search of the trouble code and the specific working generated by the readout will give some information I'm sure....Work safely please. and beware of air bags when messing with seat safety issues. If you deploy an air bag on yourself while messing with the safety controls.....you could be killed or injured....so be careful.

battery do boil, may be bad battery, check the volts 12,5 off to 14.7 max running, this car needs head gasket checked by looking to se if the coolant is too high in the bottle or too low

absolutely no idea offhand to be honest i would suspect a earth connection behind the dash somewhere as my first suspicion ,but it may pay to ask a local dealer as a dealer fitter who only works on one make of vehicle would undoubtly have more experiance of their own make of vehicle than a general mechanic like myself who will attempt anything on any vehicle but doesnt know the itty ***** gritty of that or this particular model ---but i would sort it in the end that i can say

have to replace air bag that set it off.
then should reset with code reader.

did you have a compression test run.

It is telling you that there is a problem with the air bag system and you will need to have the error code pulled with a scanner to identify what the problem is. Sometimes they will start blinking after you reach a certain amount of miles(usually 100,000) because the air bag system has to be re-inspected after a certain amount of years or mileage.

To manually check air bag codes, turn the key On and push the driver's door switch (at the bottom of the B-pillar) 5 times or more within 7 seconds of turning the key ON. Then watch the air bag light flashing codes. The air bag light will come ON for 3 seconds and then give fast flashes if there is a code. Count the fast flashes as the code. So the air bag light on for 3 seconds and then 8 flashes is a code 8 for the passenger air bag circuit. after counting code clear codes and see if they come back

Replace your air bag light. It's prob. burned out.

Take it to an independent specialist mechanic to get the codes read and a suggested plan of action

Plug in OBD2 tool and check for error codes. Check code symptoms and reset where applicable.