really, to properly diagnose this code you need an exhaust gas analyzer.
check the exhaust gas content to see if it is actually lean, or if it is in fact too rich. If it is too rich and the Vf voltage at the diagnostic connector is above 2.5V than it would indicate a problem with the O2 sensor.
If you do not have access to an exhaust gas analyzer then connect a voltmeter to the Oxy terminal and hold the engine at 2500rpm for a minute. then while it is still at 2500 observe the voltage. it should switch back and forth between less than 0.2V and more than 0.8V at least 8 times in 10 seconds. If it does not then replace the O2 sensor