[4] In August 2023, Boeing announced that the third flight—the first crewed one—would be delayed at least to March 2024 due to weakness in certain joints within the parachute system and potentially combustible materials in the wiring harnesses.

NASA announced that Boeing prepared to reassemble the vehicle for flight, following multiple checkouts, for the CFT mission in August 2020, and that new parachutes and airbags would be fitted.

[7] For medical reasons, Eric Boe, who was originally assigned to the mission in August 2018 as the pilot, was replaced by Michael Fincke on 22 January 2019.

The vehicle docked with the International Space Station and was supposed to return to Earth for a ground landing in the southwestern United States.

[15] In November 2023, NASA announced that the mission was on track for an April 2024 launch, with most of the flammable material removed from the spacecraft and a drop-test of the redesigned parachute system planned for January 2024.

[33][34] Meanwhile, in an unrelated problem, NASA and Boeing discovered a small helium leak on Starliner's propulsion system, which delayed the launch further to allow the teams to assess the situation.

[37][38] Astronauts Barry Wilmore and Sunita Williams after returning to Houston following the previous scrub, flew back to Kennedy Space Center on 28 May.

[39][40][41] In late May, the pump in the ISS urine processor assembly malfunctioned, halting the ability to convert the crew's waste back into drinkable water.

Although the Starliner spacecraft is designed to operate autonomously and these capabilities are not required in a nominal mission, these tests showed that the crew can take over many functions of the craft during an emergency.

[52] Late on 5 June, just before the crew's sleep time, flight controllers on the ground detected two more helium leaks in different parts of Starliner's propulsion system.

[57] Mission teams managed to bring back four of the thrusters by doing a series of resets and hot-fire tests on them, during which the crew manually held the spacecraft just outside the station's 200-meter keep-out zone.

Mission managers believed the failure of the thrusters could be related to input data being outside some predetermined limits rather than being a software or hardware problem, although the exact cause is unknown.

[58][54][56] Starliner docked with the forward port of the ISS Harmony module on 6 June at 1:34 pm EDT, nearly 27 hours after launch, including an hour-plus delay caused by the thruster problem.

[54] Astronauts Butch Wilmore and Suni Williams entered the station at 3:45 pm EDT, joining Expedition 71 crewmembers Jeanette Epps, Matthew Dominick, Tracy C. Dyson, and Michael Barratt of NASA, as well as ISS commander Oleg Kononenko, Nikolai Chub, and Alexander Grebenkin of Roscosmos.

The CFT astronauts were also joined by Matthew Dominick and Tracy C. Dyson to test the living conditions on Starliner with a crew of four inside.

[63] On 10 June, with all their initial Starliner testing completed, the CFT crew started working on general ISS maintenance and research activities.

Later, Wilmore worked on the maintenance of a computer connected to the Microgravity Science Glovebox, while Williams installed hardware to support a space fire investigation.

[63][64] On 11 June, the astronauts spent their time on biomedical activities, with Wilmore organizing the inventory of the Human Research Facility, and Williams working on procedures to collect microbe samples and sequence their genes.

[72] While Starliner was docked to the ISS, NASA and Boeing teams continued to assess the spacecraft's performance, especially relating to the helium leaks and RCS thruster problems.

On a 10 June update, NASA reported a fifth small helium leak in the service module, plus a new problem: an RCS oxidizer isolation valve that did not close properly.

[75][76] In July, a joint NASA-Boeing team began conducting ground tests at the White Sands Missile Range in New Mexico on an RCS thruster that had been planned to be used on a future Starliner mission.

However, when the tests were replicated on the Starliner in orbit, the same problem was not seen, and even thrusters which had previously lost significant thrust performed close to normal,[79] suggesting that the seals might not be the root cause.

After those results, at a meeting of key NASA engineers called the Program Control Board, Ken Bowersox said, "We heard from a lot of folks that had concerns."

[84] The agency also considered several return scenarios if Starliner was deemed unsafe to bring Williams and Wilmore home, including launching Crew-9 with two empty seats.

[85][86] However, NASA said that taking such an action would add additional risks as the Starliner would require a software update to allow it to fly autonomously.

[89][90][91] On 28 June, NASA announced that Starliner would not be approved to return until its thruster problems were solved, or at least better understood, or if the ISS were to experience an emergency.

After NASA decided to end the Starliner test flight without a crew on board, the company refused to answer questions from journalists, instead opting to release only brief statements.

[97] Under Starliner's original plan, the trip home from ISS would have begun with the astronauts closing the hatch and about three hours of further preparation for undocking.

[102][103] All potential landing sites were in the western United States, allowing the service module to be jettisoned for a destructive reentry over the Pacific Ocean.

[104] During the reentry, Starliner experienced two technical problems unrelated to its earlier issues: a brief glitch in its navigation system and a consistent failure to ignite by one of the 12 thrusters used to orient the capsule during atmospheric re-entry.