TOKYO (AP) -- A sensor problem was found Sunday in one of the exchanged batteries for a Japan Airlines Boeing 787 jet but did not pose a safety risk, a Japanese broadcaster reported. The Boeing Co. ( BA)jets had been grounded for four months until their safety could be reconfirmed.The problem emerged in a sensor that detects overheating in the modified version of the lithium-ion batteries used in the aircraft, NHK TV said. The batteries were encased to prevent overheating from spreading. JAL offices were closed for the weekend, and officials were not immediately available for comment. "Dreamliner" batteries overheated on planes that were in flight in Japan and parked in a Boston airport. The jets resumed commercial flights last month at JAL and All Nippon Airways.