system too lean (bank 1)

Check Engine Light (CEL) System Too Lean (Bank 1) Re: 2003 Mazda Protg5, 46,800 miles, All Stock. The CEL came on, took car to Auto Zone, got code - System Too Lean (Bank 1). I call my maintenance Mazda Dealer and they wanted over $300.00 just to change the Oxygen Sensor. I replaced the...