3Way converters working as two catalyst process: 1. Reduction and 2. Oxidation and a sophisticated oxygen storage/engine control system to convert three harmful gasses HC, CO and NOx. This is not an easy task: the catalyst chemistry required to clean up NOx is most effective with a rich air/ fuel bias. To operate properly, a three way converter first must convert NOx (with a rich air/ fuel bias), then HC and CO (with a lean bias).