In the first three months of emissions inspections in Northern Virginia, fewer than 25 percent of the vehicles tested failed to meet clean-air standards, according to a state police estimate.

Lt. Jerry Conner, administrator of the inspection program, said exact figures have not been compiled yet but a random sampling of receipts shows that most automobiles are passing the test the first time. Conner said most vehicles that were rejected needed only minor repairs, such as choke, idle speed or carburetor adjustments. He said the average cost of such repairs has been about $20, and pointed out that they usually result in better gasoline mileage.

The tailpipe tests are required by Virginia law and an amendment to the federal Clean Air Act because some Northern Virginia areas have not met federal air-quality standards for ozone and carbon monoxide levels. Virginia opted for a decentralized inspection system and in Northern Virginia, the tests began in December 1981. The requirement applies to vehicles registered in the counties of Arlington, Fairfax and Prince William and the cities of Alexandria, Fairfax, Falls Church, Manassas and Manassas Park.

In those areas, all gasoline-powered, liquid-cooled motor vehicles from model years 1975 through 1982 must pass the emissions check as a condition of registration or renewal. Air-cooled and diesel vehicles are exempt, as are trucks weighing more than 6,000 pounds. Beginning Oct. 1, new vehicles, starting with 1983 models, will be exempt for one year. About 500,000 vehicles are expected to be tested this year.

The exhaust tests are available at more than 100 state-licensed safety inspection stations in the region, which display large posters bearing the green-and-black emissions test logo.