What can cause an emissions light to come on?

What can cause an emissions light to come on?
Possibly an O2 sensor malfunction. A majority of the "Emissions" or "Check Engine" light indications are from these sensors, especially if the car still runs well. Newer cars may have two O2 sensors - one before the catalytic converter and one after. It can also be triggered by a loose or missing fuel cap or a leak in the fuel vapor recovery plumbing, neither of which would cause the engine to run poorly. The best way to find out is to have your mechanic plug in an engine computer code reader and perform a diagnostic test to see what is making the light go on.