P0171
Fuel System Lean (Bank 1)

What would cause this , a bad O2 sencer.

 

That is what it sounds like to me.

 

Could be the bank1 O2 sensor, an exhaust leak allowing air to enter the exhaust giving the sensor in incorrect reading, vacuum leak on bank1...there's a few things that'll throw that code. Believe it or not, that de-screened maf that's in your sig could even contribute to this code. (we see this code at Lexus all the time, and 9 times out of 10 it's a bad maf sensor...can't really compare Lex and GM though...)