The Federal Aviation Administration recommended late Sunday night that airlines begin visual inspections of door plugs installed on Boeing 737-900ER planes, the second Boeing model to come under scrutiny this month.

The F.A.A. said the plane has the same door plug design as the 737 Max 9, which had 171 jets from its fleet grounded after a door panel was blown off one of the jets shortly after an Alaska Airlines flight left Portland, Ore., on Jan. 5., forcing an emergency landing.

The door plugs are placed as a panel where an emergency door would otherwise be if a plane had more seats.

The federal aviation safety agency subsequently grounded the 737 Max 9 fleet and announced it was investigating whether Boeing failed to ensure that the jet was safe and conformed to the design approved by the agency.

The F.A.A. said Sunday that the door plug on the 737-900ER, which is not part of the Boeing Max line, has not yet been a problem.

“As an added layer of safety, the Federal Aviation Administration is recommending that operators of Boeing 737-900ER aircraft visually inspect mid-exit door plugs to ensure the door is properly secured,” the agency said in a statement.

The F.A.A. is recommending that the airlines using the Boeing 737-900ER immediately inspect the four locations used to secure the door plug to the airframe. The Boeing 737-900ER has over 11 million hours of operation and about four million flight cycles, according to the F.A.A.

“We fully support the F.A.A. and our customers in this action,” Boeing said in a statement.

Alaska Airlines, United Airlines and Delta Air Lines, which use the 737-900ER, each said in statements that they had already started checking the door plugs on their fleet. None of them expects any disruptions to their operations.

The incident involving the Alaska Airlines 737 Max 9 flight earlier this month did not result in any serious injuries, but it could have been far graver had it occurred when the plane was at its cruising altitude. The National Transportation Safety Board is investigating the incident in hopes of learning what caused the door plug to be expelled from the plane.

In the meantime, the F.A.A. recently ordered an initial round of inspections of 40 of the grounded Boeing 737 Max 9 planes as it works to develop final inspection instruction for the planes.