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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.”