According to the FAA, United mechanics replaced a Boeing 787's fuel pump pressure gauge on June 9, 2014, but did not inspect the work until 19 days later.
In the interim, the plane made 23 domestic and international passenger flights, including two after the FAA notified United of the lapse, the agency alleges.
"We immediately took action after identifying the issue and are working closely with the FAA in their review," United told ABC News, adding that safety is the airline's "top priority."
"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."
ABC News' Whitney Lloyd contributed to this report.