If you were going to build a system to check the effectiveness of automobile catalytic converters on cars, what substances would you want to look for in the car exhaust?

Oxides of nitrogen and hydrocarbon

Explanation:

An automobile catalytic converter is a substance that catalyses redox reactions that convert toxic gases emanating from an automobile engine to less toxic gases.

The substance work by converting carbon monoxide (CO) and hydrocarbons to carbon dioxide and water and oxides of nitrogen to nitrogen and oxygen gas respectively.

Hence, in order to check the effectiveness of a catalytic converter, unburned hydrocarbon and oxides of nitrogen are substances to watch out for in the car exhaust. None of these gases should be found in an exhaust with effective catalytic converter and vice versa.