Boeing on Sunday said some of its 737 planes, including many 737 Max aircraft, may have faulty parts on their wings. It's the latest problem Boeing faces as it tries to get its most important and popular airplane, the grounded 737 Max, back in the air.

Working with the Federal Aviation Administration, Boeing said it has reached out to airlines that fly 737 planes, advising them to inspect their slat track assemblies on Max and NG aircraft. The 737 NG series includes the 737-600, -700, -800 and -900 planes.

Leading edge slats are an aerodynamic control surface that extend from the front of the wing. Some the tracks may not meet manufacturing standards and may need to be replaced, Boeing and the FAA said. They said if the parts are found to be defective, airlines should replace them before returning the planes to service.

The faulty parts could fail prematurely or crack. The FAA said a part failure would not bring down a plane, it could damage an aircraft while in flight.

Boeing has sent out a service bulletin and the FAA will issue an airworthiness directive requiring airlines to inspect and repair its slat track assemblies within 10 days.

The company discovered the problem Friday, when Boeing was meeting with the parts supplier. Boeing employees noticed some of the parts were not heat treated, which led them to believe there might be a safety issue.