My check engine light came on on my 2005 Chevrolet Monte Carlo LS & the code was p0171. I took it to a mechanic & was told that I need to replace my upstream O2 Sensor. Once replaced, the code was reset but, not even a mile down the road, the light came back on. I took it elsewhere & was told that, had it been my O2 Sensor, the code would've specified O2 sensor but it was just reading bank 1 too lean which means that possibly a cracked or leaking hose somewhere around the intake. I'm not sure what to do...Help!

 

this car only runs 1 pre cat 02 sensor, not one for each bank like other cars. Therefore it could be lean on both banks and you have no way to check with a scan tool.

things that cause lean fuel trim are;

vacuum leaks...which can be ruled out if you have a scan tool
exhaust manifold leaks
bad maf sensor
low fuel pressure

take it back to the first mechanic and tell him the problem isn't fixed code has returned. See what they want to do for you. A good shop will correct their mistake. you should get at least free diagnosis the second time and cost for the part.