GM-GMC: fuel guage 200 gmc sonoma, gmc sonoma, fuel guage

Question
my fuel guage is unreliable. one second it will read full then it will bounce down to empty and the low fuel light comes on. it seems to go down to 3/4 then all of a sudden it goes to empty. is this the guage or the sending unit in the tank?

Answer
It could be either, the only way to tell for sure is to monitor the signal from the in tank sender to the dash gauge with a voltmeter to verify wether the voltage is changing causing the gauge to react to this signal(sender problem) or if the voltage signal stays constant/steady and the gauge is still bouncing(gauge problem)