I just recently bought a 2004 GMC Sierra 1/2 ton pickup (I leased it brand new for three yrs), 37,280 miles, 5.3L V-6.. I have never had one problem with it until now. A few days ago, a message flashed stating that I needed to service my 4 wheel drive (it hasn't said anything since then). Then just yesterday, I got into my truck to go to work, and the engine came on and stayed on. Half way down the highway, I received a message saying to tighten my fuel cap (which I did when I got to work). After getting out of work, the engine light was on again and stayed on, and again today. My truck is not running rough, and has never stalled out, and it runs fine going down the road like it always does. Just can't figure out why the engine light came on. Could it be a oxygen sensor going out, the computer acting up, or could I have gotten a hold of some bad gas? I'm leaving on a trip in a few weeks and really can't afford a big repair bill. Any suggestions is greatly appreciated.