As Boeing is trying to get its 737 Max planes back in the air after they were grounded following two fatal crashes tied to an automatic safety feature, a new issue has come up with some of its jets. The airplane maker said Sunday (June 2nd) that some of its 737 planes, including many of the 737 Max jets, may have faulty parts called slat track assemblies on their wings.

Boeing has advised airlines that fly 737 Max and 737 NG series planes to inspect the parts and to replace them if they’re found to be defective before returning them to service. They said some of the tracks may not meet manufacturing standards and could fail early or crack, which the Federal Aviation Administration said wouldn’t cause a plane to crash, but could damage it while in flight. Boeing said it discovered the problem on Friday.