I have a strange one. I have 2002 Camaro (v6 3800 series II) It gave me the "service engine soon" light a few weeks back. I checked the error code at Autozone and they said it was P0174 bank 2 too lean. I filled it with better gas and thought it went away. Nope, it came back shortly thereafter! I looked at some forums and cleaned the MAF and looked for anything out of place. I haven't checked for vacuum leaks or replaced any O2 sensors, because I have no other symptoms that I can find. The error would come and go. Today I realized, when I have a full or close to full tank of gas, it goes away, but will come back when I get close to half a tank. Does anyone have any clue what this might be?

 

If it was a 5.7 liter first shot would be the O2 sensor, swap left with right or replace both but I have no 3800 experience. Same with The 1/2 tank, if it was an LT1 could be bad overheating pump causing low pressure, but IMO 1/2 tank should be enough to cool it down. You would need a fuel pressure gauge to check that out.