The Marine Corps has grounded four Harrier jets of its mainstay fighter-bomber force because a key component of the aircraft has failed more quickly than expected, a Marine official said yesterday.

The component, which helps steer the plane in routine flight, has been in short supply because of the failures and a government order barring additional purchases from its sole manufacturer, the Northrop Corp., after company officials admitted faking some key component tests, said Col. James G. Hart, the Harrier program manager.

In addition to the four short takeoff-and-landing aircraft already grounded, 14 more could be grounded over the next 12 months if there are no new spare parts purchases, Hart told the House Government Operations subcommittee on legislation and national security.

But he added that the Navy, which manages the plane's procurement, recently won a Defense Department waiver of the proscription against further purchases from Northrop so it could replenish its supply. The Navy has also begun what Hart called a "lengthy and expensive" effort to obtain the parts from another defense contractor.

The Marine Corps has 180 Harriers to support operations by Marine ground troops. An unspecified number of the planes have been deployed with U.S. forces in the Persian Gulf crisis.

Rep. John Conyers Jr. (D-Mich.), the committee chairman, said on NBC's "Today" show yesterday that "there's no question" U.S. forces in the region were at risk from the Harrier problems, and that some of the planes there had been "pulled gradually from duty."

Navy officials said the groundings involved only those planes deployed elsehwere, however, and Conyers's aides said later that they had no information to dispute the Navy claim.

The component, known as a "rate sensor assembly," was ordered by the Navy to aid manuevering of the AV-8B Harriers after an earlier version of the plane, the AV-8A, experienced an unusual number of crashes leading to 24 pilot fatalities.

Howard H. Hyde, a former Northrop engineer sentenced to Lompoc Federal Prison for his role in the testing fraud, testified that he helped prepare false reports on the sensors' performance because the company was eager to meet a tight schedule and lacked funds to buy the appropriate test equipment or arrange for independent certification.

Hart said the Harrier sensors on average were failing after four years, roughly two-thirds of their expected lifetime. "Reports from the field indicate that transformers have burned on circuit cards, capacitors have not been epoxied or properly soldered, and resistance to vibration is poor," Hart said.

Northrop also falsified test data for a component of the Air Force's air-launched cruise missile, equipped with nuclear warheads, said Hyde, who was escorted to the hearing by federal marshals. However, Air Force missile engineer Jack Jones testified that no cruise missile failures had been connected to the fraud.

Hyde said he had no evidence that Northrop officials other than his immediate superior were aware of the fraud, and apologized for not "thinking more broadly" about placing pilots' lives at risk. The company pleaded guilty last March to 34 fraud charges related to the Harrier and cruise missile programs, and paid a $17 million fine, in exchange for the government's dismissal of investigations into potential Northrop fraud under other major weapons contracts.