trouble code p0300 with a p0156 following. help anyone?
 

my engine light came on and read code p0156. so i replaced oxygen sensor cause it said bank 2 sensor 2 was bad and truck ran fine for the day and my engine light went off on its own. later in the day, engine light came on now showing p0300 with the p0156 following? needing help. 2004 silverado 5.3 2wd truck