Q: Vehicle won't pass emissions test

Emissions tests analyze three main exhaust gas components: hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2). In some places, they also look at oxides of nitrogen (NOx). Excessive CO is the result of a rich fuel mixture and can only occur when combustion has taken place. For example, when a cylinder misfires, CO cannot be produced since the hydrogen and carbon in the fuel have not been separated. NOx on the other hand, results from combustion temperatures that are too high. A failed EGR valve is often the cause of excessive NOx readings. You may have two separate issues, or your catalytic converter may have failed. Your Check Engine Light is likely on, which may provide a clue. I suggest having a trained professional, such as one from YourMechanic, give a complete inspection of the vehicle and determine what needs to be repaired.