United Airlines operated a plane that was "not in airworthy condition" during 23 domestic and international flights, the Federal Aviation Administration said.
On Tuesday, the FAA alleged that United flew the unsafe aircraft for a few weeks in June 2014 and is seeking a $435,000 civil penalty against the airline.
According to the FAA, United replaced a faulty fuel pump switch on a Boeing 787, but did not inspect the aircraft properly after that.
"Maintaining the highest levels of safety depends on operators closely following all applicable rules and regulations," FAA Administrator Michael Huerta said in a statement. "Failing to do so can create unsafe conditions."
United is seeking a meeting with the FAA to discuss the allegations, the agency said.
"At United, the safety of our customers and employees is our top priority," a United spokesperson said in an emailed statement. "We immediately took action after identifying the issue and are working closely with the FAA in their review."
This story has been updated to reflect comment from United.