The Federal Aviation Administration says United Flight 409 bound for Seattle returned to the airport at about 9:15 a.m. Sunday after reports of possible smoke in the cabin and cockpit.
No injuries were reported.
The emergency landing was the second in as many days at Newark. On Saturday night a United flight bound for Berlin had to return to the airport after experiencing engine problems.
On Sunday the FAA said that one of the engines on the Berlin-bound flight apparently overheated on takeoff. An agency spokesman says that contrary to published reports, the aircraft didn't blow a tire during takeoff.