In the February 1997 issue of Import Service, we began the multi-part “Gas It” series on exhaust gas analysis. We focused our discussion on carbon monoxide (CO), the “rich indicator” and hydrocarbons (HC), the “incomplete combustion indicator.” This month we will look at the additional information to be gained from consideration of a third gas, oxygen (O2), the “lean indicator.” We’ll use a question and answer approach to get our points across again this time.