The launch of the first crewed flight of a new spacecraft to send humans to the International Space Station has been delayed until at least May 17 because of a technical issue.

The Boeing Starliner was set to launch Monday night from Cape Canaveral spaceport in the southeastern U.S. state of Florida for a weeklong test flight to the ISS. Crew members Barry Wilmore and Sunita Williams had just entered the capsule when ground officials scrubbed the launch, citing a faulty oxygen pressure valve on the second stage of the Atlas V rocket.

Officials with United Launch Alliance, the company that built the rocket, said engineers heard an audible buzzing sound from the valve during the countdown.

ULA, Boeing and the U.S. space agency NASA had initially hoped to reschedule Starliner’s launch for this Friday, but NASA said late Tuesday that ULA had decided to replace the valve, believing it had exceeded its usefulness.

The rocket and its attached spacecraft will be rolled from the launchpad into a special assembly building Wednesday to repair the problem and conduct further inspections.

Wilmore and Williams, veteran NASA astronauts, will test Starliner’s systems during the coming mission to determine if NASA can certify Boeing to conduct routine flights to and from the space station for the agency.

If the Boeing test is successful, it will become the second reliable option for human space flight along with Elon Musk’s SpaceX Dragon capsule. SpaceX has been shuttling NASA astronauts to and from the ISS since 2020.

The Starliner program has experienced years of setbacks and delays that have added more than $1.4 billion to the cost of the program.

During a 2019 uncrewed test flight, the capsule was on an incorrect trajectory and returned without reaching the ISS.

Another launch was postponed in 2021 because of a valve problem. An uncrewed capsule reached the ISS in May 2022. But the spacecraft has experienced several problems since then, including the discovery of flammable tape in the cabin and weak parachutes.

Some information for this report came from Reuters.