Major Rebecca Heyse of Luke Air Force Base confirmed a F-35 pilot had a physiological event on Monday that is similar to previous hypoxia-like events.
The pilot activated the backup oxygen system on the plane and headed back to base.
Medical personal could not determine a hypoxia event.
After the flight, it was determined there were some irregularities with an oxygen valve on the pilot's flight helmet. That valve was replaced and the pilot has not had any issues since.
Pilots in the F-35 have had several hypoxia-like events that caused the planes to be grounded.
PREVIOUSLY: F-35 flights remain paused due to oxygen issue
This is the first time Luke officials have found a specific issue with an oxygen system. Pilots will continue to be monitored.