California is considering adopting an enhanced Inspection and Maintenance (I&M) program (commonly referred to as Smog Check II) beginning with the 1996 calendar year. This program will utilize a targeting scheme to identify vehicles likely to be high emitters and send these vehicles to centralized testing facilities. The remaining fleet of vehicles will be sent to decentralized testing facilities. At these facilities, vehicles will be subjected to steady state loaded mode dynamometer based tests. Simultaneously, all 1996 and later model year passenger cars, light- and medium-duty trucks sold in California will be equipped with an On-Board Diagnostic (OBDII) system. This system is designed to monitor critical emission related components and activate a Malfunction Indicator Light (MIL) when a failure or a drift in calibration is likely to cause emissions to exceed 1.5 times the vehicle certification standards. The main objective of this paper is to ascertain what percentage of the emission benefits are attributable to either I&M or OBDII in order to assist regulators in making near term programmatic decisions. This paper also addresses the potential emission benefits of incorporating a radio transponder into the vehicle's OBDII system that is capable of transmitting fault codes when queried, resulting in prompt identification and repair.