Laboratory and inspection lane tests of a variety of 1981 and later model year passenger cars and light-duty trucks indicate that the test procedures currently specified for IM240 and Acceleration Simulation Mode (ASM) testing in vehicle inspection and maintenance (I/M) programs provide inadequate preconditioning for many vehicles. Changes to both the length of time and the speed-time profile of the preconditioning period are required to minimize false failures while minimizing the time required for testing.Under the current IM240 procedures, test results from the first 93 seconds of the test, called “Phase 1,” are ignored when the composite test results exceed the applicable standards. Phase 1 operation is presumed to be adequate for preconditioning and the pass/fail decision is made based on the last 146 seconds of the test, which is called “Phase 2.” Data from the Arizona I/M program and laboratory tests indicate that most vehicles are not thoroughly warmed-up by running Phase 1 of the IM240 test if they have been waiting in a queue for 15-30 minutes prior to the test. However, adequate warm-up is more often achieved if the order of the two phases is reversed. This will occur because the 146 phase not only is longer, but also contains significantly higher speeds, which are more effective in warming-up the engine and emissions control system. An even more effective test procedure change would be to eliminate the first phase of the test and replace it with two 146-second phases. For vehicles that fail the applicable standards after running two 146-second phases, a comparison of the second-by-second data collected during each phase would indicate whether further warm-up is required, as will be the case in certain circumstances (e.g., following a longer wait in a queue).Under the current procedures for a single mode ASM test, a maximum of 175 seconds of operation is allowed prior to the final chance to pass the emissions standards. Preliminary testing indicates that this level of preconditioning is marginally adequate in a centralized I/M environment as long as the vehicle sits in a queue for less than 30 minutes. In a decentralized I/M environment, where a vehicle may sit for several hours before being tested, longer preconditioning will often be required. Data collected during laboratory testing indicate that additional preconditioning time should be allowed if emissions concentrations are still declining after 175 seconds. In addition, it is apparent that some vehicles exhibit unrepresentative emissions performance under certain ASM test conditions. For vehicles that initially fail to meet the emissions standards, the introduction of a significant speed variation may be required to ensure that the vehicle is not in an open-loop operating mode induced by extended steady-speed operation.