1998 Nissan Altima rotten-egg" smell

The "rotten-egg" smell is caused indirectly by the catalytic converter. The main cause is a rich mixture condition from the engine. Check the catalytic converter to make sure it is not "plugged" or restricted. Your car is equipped with a computer controlled engine management system. You can scan your computer for fault or error codes with a hand held scanner. The information will pinpoint the mal functioning components.