CAPE CANAVERAL - NASA on Monday (local time) cleared space shuttle Endeavour for a second launch attempt on May 16 to deliver a pioneering physics experiment and spare parts to the International Space Station.

NASA was nearly four hours away from launching Endeavour on its 25th and final mission on April 29 when a heater in an onboard power system failed, prompting managers to stop the countdown.

Engineers traced the problem to an electronics box, which was replaced over the weekend. Technicians also installed new wiring from the heater to the new box in case the short circuit stemmed from outside the device.

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The repair involved retesting other shuttle systems, including life support, environmental control and propulsion that draw power through the same electrical switching box.

The heater keeps hydrazine fuel used in the shuttle's power unit from freezing and potentially bursting in space. The power unit, one of three onboard the shuttle, provides hydraulic power to move rocket engine nozzles, adjust flight surfaces and lower the shuttle's landing gear.

"We've checked and wrung out the whole system," said Mike Moses, who oversees NASA's shuttle launch management team. "We now have extremely high confidence the problem is no longer on the ship or in any of the electronics."

Delays launching Endeavour on NASA's next-to-last shuttle flight will postpone the final flight of the 30-year-old program.

Atlantis, which will be carrying a last load of supplies for the station, had been targeted for launch on June 28. The mission likely will be delayed about two weeks, Moses said.

NASA is ending the shuttle program due to high operating costs and to develop new spaceships that can travel beyond the station's 220-mile (350 km) high orbit.

Endeavour's primary payload is the $2 billion Alpha Magnetic Spectrometer, which is designed to probe high-energy cosmic rays for signs of dark matter, antimatter and other exotic phenomenon.

The shuttle and its six-man crew, headed by commander Mark Kelly, are slated to spend 16 days in orbit to help prepare the station for operations after the shuttles are retired.