Rep. Stephen Horn (R-Calif.), who has vexed federal agencies with his Y2K "report cards," issued final grades yesterday. The Clinton administration received a "B+," up from its first grade of "D" handed out in July 1996.

"We have come a long way," Horn said.

But Horn, even as he handed out passing grades to the 24 largest agencies, found fault with 18 programs, including the nation's air traffic control system, saying they "remain at risk of failure."

Administration officials disputed Horn's assessment, with several saying his data was based on Nov. 15 reports to the Office of Management and Budget that are already outdated, given the rush to wrap up Y2K fixes before Dec. 31.

With more than 99 percent of the government's critical systems repaired, "we have every reason to expect that the government will be fully compliant in time for the date change to the year 2000," OMB spokeswoman Linda Ricci said.

At his news conference, Horn said Y2K would not threaten air safety--he has bought tickets to fly from Los Angeles to Dulles on Dec. 31--but called the air traffic control system "one of our main worries. . . . Are they ready? They aren't. And hopefully they will be. They've been playing catch-up."

Joel C. Willemssen, a top Y2K expert at the General Accounting Office who has assisted Horn in the grading of agencies, said the Federal Aviation Administration still needs to finish testing integrated systems and some communications systems.

But FAA spokesman Paul Takemoto said, "The FAA is ready to go." The agency wrapped up the bulk of its Y2K work on June 30 and FAA Administrator Jane Garvey plans to fly across the nation on New Year's Eve to demonstrate her confidence in the system.

Since June, Takemoto said, the FAA has retested one system, Terminal Doppler Weather Radar, which warns airports of wind shear and other dangerous conditions. Some other systems have not undergone integrated tests because they are stand-alone computers, he said.

In addition to air traffic control, Horn said other programs facing Y2K risks include federal food stamps, child nutrition programs, the student loan program, Medicare and Medicaid and public housing programs.

Virtually all of these programs rely on states and localities to administer services and benefits. Ricci said federal agencies are "working with and urging the states to move as quickly and efficiently as they can" to resolve any remaining Y2K concerns.

The Year 2000 computer problem, popularly known as Y2K, stems from the use in many systems of two-digit dates. A number of experts have worried that such systems might misinterpret "00" as 1900, not 2000, and shut down or otherwise malfunction.

Horn gave A's to 15 federal agencies, B's to five and C's to three. The Justice Department got the lowest mark, a D. Horn faulted Justice because it has three systems to fix; he said the department's contingency plan was "worthless" because it has not been tested.

Assistant Attorney General Stephen R. Colgate called Justice's grade "undeserved." The department does not face a Y2K problem but "a matter of logistics of getting compliant workstations over to foreign countries" where agents have offices, he said. The overseas installations will be completed in December, he said.

Justice's contingency plan is a compilation of each bureau's back-up plans, which have already been separately tested, Colgate said. As a further test, Justice will conduct a Y2K drill at its command center on Dec. 8 and 9.

CAPTION: Reps. Stephen Horn (R-Calif.) and Connie Morella (R-Md.) discuss how agencies were graded on Y2K preparedness.