Has it always been on? the vehicle may have previously been into an accident causing the airbags to deploy.  Airbags are fairly expensive and many used car dealerships do not replace them once they have been in an accident. they could have reset the sensors causing the light to come off for a while. You may not even have an airbag in there.

Another problem may be the wiring. If you travel down bumpy or rough terrain you can easily cause an air bag wire to become unplugged. Many vehicles have the air bag sensors below the vehicle near the suspension and rack and pinion which is a bad spot for them considering that area of the car takes a lot of hits. did you hit any curbs or big bumps lately?

check if the air bags are in fact on. you usually need the key to turn off the airbags.

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