As mentioned, you should first check for codes if the engine light came on. Black stuff in the exhaust I wouldn't contribute to the converter, but with the engine producing the exhaust. Which by the way your converter might be shot because of the problem anyway, but a new converter would go bad soon, so I would concentrate on solving the problem, then replace the converter. The source could be a wide variety of things, but my 2 pennies worth of guesses would be that the electrical emmisions is not working correctly, the ECM is malfunctioning, or perhaps the fuel injection system is malfunctioning.