All Nippon Airways has reported a "minor" problem with a modified Dreamliner undergoing a test flight.
The Japanese carrier said a damaged electrical distribution panel, unrelated to the battery system that grounded the entire Boeing 787 fleet in January, was to blame.
ANA said a nut connecting an electrical wire to a power distribution panel had not been tightened properly by a Boeing maintenance worker, according to the Financial Times.
The panel was left blackened after the nut vibrated and overheated due to friction during a test flight.
The pilot was alerted to the problem when an error light flashed in the cockpit.
A spokesperson for ANA said the airline was not obliged to alert safety regulators about the incident - described as "a minor issue that is also found on other types of aircraft" - but decided to do so anyway on May 4.
The news comes a day after Qatar Airways became the first airline to resume Dreamliner flights to the UK.
Qatar’s B787 flight from Doha touched down at Heathrow on Wednesday morning – the first 787 service to land at the airport since January due to problems with onboard batteries overheating.
Ethiopian Airlines had been due to be the first carrier to resume Dreamliner flights to Heathrow last week but changed its plans at the last minute and is now scheduled to fly the aircraft from Addis Ababa to Heathrow from June 16.
Before then, Air India is due to start daily Dreamliner flights from Delhi to Heathrow from May 22. United is also scheduled to put the 787 on its Houston to Heathrow route from June 10.