is it still flashing? how long dose it do it? mine flashed b4 but only about 20 seconds or so, and went away. soemtimes they do strange things for no reason at all. but if its still doing it call a nissan dealer or just take it to them and point at it
if your altima is a first gen, push the dome light switch in door jamb (plunger type switch) like 10 times in 10 seconds (cant remember actual times that you press) That is supposed to reset airbag computer, I have seen it work on my buddy's car..