The Clean Air Act of 1990 requires on-board diagnostics (OED) capabilities on all new vehicles. These diagnostic systems monitor the performance of engine and emission system components and inform the vehicle operator when component or system degradation could significantly impact emissions. Acceptable operation of the monitor requires proper treatment of system variables. Fuel composition is one of many possible variables that must be considered for monitoring components directly in the exhaust stream.Recently, the octane enhancing, emissions reducing additive methylcyclopentadienyl manganese tricarbonyl (MMT) was reintroduced into unleaded gasoline in the U.S. Prior to reintroduction, the additive underwent extensive testing to demonstrate that use of MMT does not adversely affect vehicle emissions or the operation of emission systems such as OBD. However, questions have been raised about the influence of the additive on OBD systems. The results from additional testing, carried out as part of the ongoing evaluation of the additive, are reported in this paper. The results of this testing, including studies performed on a production OBD system, demonstrate that the use of the fuel additive MMT does not adversely affect the ability of OBD systems to identify degraded emission components. In addition, this study has led to a better understanding of the design and operational parameters behind a current OBD catalyst monitor system.