Look at my post in general discussion. My daughter ran her Titan through a ditch, not hitting the grill or front bumper. From what I have learned, all you have to do is really jar the the sensor mounted on the radiator support. Once it goes into alarm mode, it has to be replaced.
After you replace the senser--with the battery disconnected and key off until finished-- you have to reset the airbag system yourself ( found on this site ) or at the dealer to make it work correctly. Been there, done it. Sensor is roughly $150. from dealer. Don't buy one used, you don't know the condition.