TOKYO (AFP) ― Japan Airlines has grounded another one of its Dreamliner jets after “white smoke” was seen outside the cockpit window during maintenance, a year after the aircraft suffered a months-long global grounding over battery problems.

The carrier said that a technician at Tokyo’s Narita airport, who was working on the parked plane before its departure to Bangkok Tuesday afternoon, first noticed what appeared to be smoke outside the cockpit window and then a battery system warning.

An investigation found that one of the eight lithium-ion cells in the plane’s battery system had leaked, but its safety valve, which is designed to release excessive pressure, was properly open.

“The temperature of the cell was high. We believe it caused ‘white smoke,’ which could be smoke or vapor,” a JAL spokesman said.

The airline said it replaced the grounded plane with another Dreamliner, which left as scheduled, carrying 169 passengers and crew.

“We are making sure of the safety of every plane before its departure. We will continue regular flights (with Dreamliners),” said a separate JAL spokesman, Norihisa Hanyu.

In response to the incident on Tuesday, U.S.-based Boeing said the “improvements made to the 787 battery system last year appear to have worked as designed.”

Boeing admitted in April that despite months of testing it did not know the root cause of the battery problems, but rolled out modifications it said would ensure the issue did not recur.

The measures comprised redesigning the battery and charger system and adding a steel box to prevent burning.

Since then, Dreamliners have experienced a series of minor glitches, including a fault with an air pressure sensor and the brake system.

In October, unflushable toilets caused JAL pilots to turn their plane around just after it left Moscow bound for Tokyo.

JAL’s domestic rival All Nippon Airways also said Wednesday it would continue flying the plane ― the pair are the aircraft’s two biggest customers and have invested heavily in its success.

The incident was the latest for the Dreamliner since the trouble-plagued aircraft returned to service in the middle of last year following a months-long worldwide grounding.

The move was linked to a string of incidents, including a fire aboard a parked Dreamliner, which damaged the fuel-efficient jet’s reputation and that of its manufacturer Boeing.

Investors appeared unfazed with JAL’s Tokyo-listed shares closing up 1.33 percent at 5,300 yen, while ANA was up 0.89 percent to 225 yen, tracking a rise in the broader market.

Shares in the plane’s Japanese battery supplier GS Yuasa slipped 0.68 percent to 582 yen.

The Kyoto-based battery maker said it was “cooperating with Boeing and Japan Airlines on finding the cause” of the latest battery malfunction. It declined to elaborate further.

Hideo Inagaki, a former JAL maintenance specialist and chief researcher at Japan Aviation Management Research, said the carbon-fiber bodied plane’s glitches are little surprise given its state-of-the-art technology.

“Planes are very complicated machines and whether reported or not, they often experience glitches,” he told AFP.

“The point is whether (Boeing) is appropriately handling these new issues.”

The U.S. Federal Aviation Administration said it was working with Boeing and the Japan Civil Aviation Bureau to investigate, while the U.S. National Transportation Safety Board said it was ready to assist Japanese authorities.

Repeated efforts to get a comment from the Japanese aviation bureau’s safety unit were unsuccessful.

The number of 787s in operation worldwide had more than doubled from the 50 in service when the plane was grounded last year.