If the eror code is accompanied by error codes related to aging sensors replace the sensors indicated by the error codes, clear the error codes, and resume driving the car. If the code comes back the converter is worn out (unable to store oxygen) or perhaps has suffered some internal mechanical failure that has affected its operation.

Converters are warrantied by federal law for IIRC 7 years and 80K miles -- this should be covered in one of the books that came with the car -- so you should get the car to a dealer for 1) confirmation the converters are covered; and 2) get the bad one replaced.

I have nursed my Boxster's sub-par converter for years now running a top tier (mostly Shell, but Chevron works too) gas and buying from a busy station, and once every two years (but not always, sometimes I get lazy or the car is just behaving ok... switching away from 'supermarket' gas to a name brand gasoline really keeps the eror code at bay) before an emissions test running a bottle or two of Techron through the gas tank.

Keep the plugs reasonably fresh -- replace every 60K miles -- replace engine air filter every so often, avoid overfilling the engine with oil, and then drive the car.

Oh, I found pushing the engine a bit harder -- nothing harsh just rev the engine a bit higher, use a bit more throttle -- seems to help forestall the CEL and error code. It still comes on though almost always in cooler weather.