Toyota Repair: Trouble codes, toyota corolla, gas mileage

Question
My 2000 Toyota Corolla has a trouble of P0171 system to lean bank 1.  My question is where can I find out what might cause this and how to repair it.

Thanks

Answer
O2 sensor, 1st one on exhausts leaving manifold maybe bad, you will have to take a chance and spend the 180. more or less or take it in and have it checked by Toyota.  When a sensor reports to the Ecu a lean condition  it starts putting the fuel trim on the rich side, Gas mileage gone down? oh check under your air cleaner it's time for the rodents to move in getting that time of year, nuts , you name it I seen it all under that filter even baby squirrels, alive. Use Toyoa parts aftermarket sensors are junk, I take more of these off to fix the same thing they started with and part and labor are for 1 year.