One of the best techniques for diagnosing check engine lights or engine problems especially for lean or rich engine diagnostic trouble codes (DTC's) is to examine and interpret Fuel Trim data. This video will help the home mechanic learn how to use a very basic scan tool to develop powerful diagnostic capabilities and prevent changing parts unnecessarily.
Part 2 is here: https://www.youtube.com/watch?v=cARQ0jZZ4Qc
For more advanced Fuel Trim info- see my video on adaptive fuel learn here: https://www.youtube.com/watch?v=ILm8sJrL2R0
Secret Of Engine Problem Diagnosis- Fuel Trims Pt.1