Pursuant to a congressional request, GAO provided information on the 50-percent penalty, also called a discount, that states suffer under the Environmental Protection Agency's (EPA) Clean Air Act enforcement for operating vehicle emissions inspection and maintenance (I&M) test-and-repair networks instead of test networks.

GAO found that: (1) EPA issued the 50-percent discount rule after it found that test-and-repair networks were less effective in controlling motor vehicle emissions; (2) since issuing the rule, EPA has not conducted any studies to obtain new data on the effectiveness of test-and-repair networks; (3) 14 of the 23 states required to implement enhanced I&M programs have not conducted studies on the 50-percent discount, mostly because they implemented, or intended to implement, test-only programs; (4) the other 9 states did not conduct studies to assess the relative effectiveness of the two kinds of I&M networks; (5) California studied relative effectiveness and concluded that there was little difference between test-and-repair networks and test-only networks; (6) Georgia is conducting a study, and the preliminary findings lead Georgia to believe that it should suffer only a 35- to 45-percent discount for its hybrid I&M program; and (7) Congress enacted legislation to eliminate automatic discounts based on the type of testing network and allow states 18 months to study their I&M programs' effectiveness. GAO also found that EPA has allowed the states great flexibility in implementing enhanced I&M programs by: (1) providing I&M system models; (2) establishing credits for test-only networks, for states that require that only certified mechanics perform repairs, and for less costly testing systems; (3) relaxing performance standards for states that can still meet their overall targets for reducing emissions.