SpaceX is replacing two engines on its Falcon 9 rocket that will soon carry four astronauts to the International Space Station. The change is being made after SpaceX found a substance in the engines that could have caused them to start earlier than planned.

SpaceX found the substance after one of the company’s launches halted just before liftoff in October. One of SpaceX’s Falcon 9 rockets, tasked with launching a new GPS satellite for the Space Force, shut itself down just two seconds before takeoff. “It was a good abort,” Hans Koenigsmann, vice president of build and flight reliability for SpaceX, said during a press conference. “It did exactly what we programmed it to do.”

Today, SpaceX revealed the rocket had automatically stopped its own launch after two of its nine main Merlin engines tried to start too early. Starting up too early could have caused something called a hard start, where the engine’s various propellants and liquids are combined in the wrong order. SpaceX’s Falcon 9 shut itself down before that happened, but such a hard start could have damaged the hardware. “It’s not necessarily bad, but in most cases, you know, it rattles the engine,” Koenigsmann said. “And it may cause, you know, a little bit of damage on the engine. In extreme cases, it may cause more damage to the engine.”

After the abort, SpaceX inspected the engines and found a reddish lacquer substance blocking a relief valve in each model. The lacquer, analogous to nail polish, is used to treat surfaces, likely leftover from when the engines were being built. Koenigsmann noted that it came from one of SpaceX’s vendors that works on the engine, though he didn’t name the company. “It could be that person is now more generous with cleaning fluid or anything,” Koenigsmann said. “It’s a little bit hard to figure this out.”

SpaceX says it’s working with the vendor to implement corrective actions and that the company will be inspecting all of the hardware more closely in the short term. After the abort, SpaceX also examined the engines in its fleet that will be used for upcoming launches. They looked for any exhibiting this early startup behavior and singled out five various engines on three different rockets. Two were on the Falcon 9 rocket that will launch three NASA astronauts and one Japanese astronaut in November, a mission called Crew-1.

After this testing, SpaceX opened up all of the engines and found the same lacquer inside. SpaceX also noted that the lacquer was found mostly on newer engines that had been built recently, not the engines that have already flown to space and back. Now, the company is in the process of swapping out the engines ahead of Crew-1, which is tentatively set for November 14th at 7:49PM ET. “Two, three days is roughly what you need to take it out,” Koenigsmann said regarding the engine swap. “And then another two, three days to put it back in again.”

Ultimately, SpaceX won’t fly Crew-1 until a few days after the company launches a joint European and American satellite called Sentinel-6, which is designed to map the world’s oceans. SpaceX found an affected engine on the Falcon 9 rocket for that flight and had to swap it out, too. NASA wants to see how that launch goes first before it makes the final decision about flying its astronauts on SpaceX’s Falcon 9. The Sentinel-6 launch is currently slated for November 10th out of Cape Canaveral, Florida.

Meanwhile, the four astronauts assigned to Crew-1 have started their quarantine process, staying mostly at home with their families before heading into a stricter quarantine two weeks ahead of flight. NASA has also been keeping them informed about the engine swap and all the details leading up to launch. “They’ve been in a lot of the telecoms and listening to what’s going on with the vehicles,” Steve Stich, the program manager for NASA’s Commercial Crew Program, said during the press conference. “We have a little bit more work to do on this engine and offline… but I think we see a pretty good path to get to flight and we’ll fly when we’re ready.”