The city bus has been targeted for special emission regulations. This examination of the practical problems of day-to-day use of a Catalytic Trap Oxidizer was undertaken. Installation problems and vehicle adaptation are discussed together with safety requirements. The initial concept of muffler replacement CTO's was found to be practical but the logistics required a redesign of the unit for more practical fit and service. A failure of the initial system demonstrated the need to design for safety. A second design, simpler to fit and with safety aspects foremost, was also examined. The effects of the CTO system on vehicle performance, smoke and odor are given, together with details of the work carried out to practically achieve the positive benefits of the CTO, smoke and odor control, in a real life application.