Metro said Wednesday that it has determined what caused the door on one of its 3000-series rail cars to malfunction, inspected the fleet, and returned the roughly 300 older-model cars to service.

The agency tweeted that: “3K rail cars have been cleared to return to service today. No further service impacts are expected.”

The move comes a day after Metro pulled the 3000-series cars from service after a set of doors opened on an Orange Line train as it left the Dunn Loring station Sunday. A rider caught the incident on video.

Metro General Manager Paul J. Wiedefeld said Wednesday at a news conference about the upcoming closure of six Blue and Yellow stations, that an initial review showed there was an electrical component in the 3000-series car that “didn’t perform the way it was supposed to” and the doors opened.

He said 26 cars in the agency’s fleet of roughly 1,200 rail cars have a similar type of component and they have been removed from service. Wiedefeld said the parts on those 26 rail cars will be replaced.

Wiedefeld said the train involved in Sunday’s incident kept moving because the component didn’t work and the train operator didn’t know the door was open because “the electronics didn’t work the way they were supposed to.”

The Washington Metrorail Safety Commission, which is responsible for safety oversight of the transit agency, has launched an investigation into the incident.

This isn’t the first time Metro has had problems with doors opening while a train is moving.

In 2015, the agency’s 4000-series cars had to be pulled from service for a door problem. A year later, the 4000 series was again pulled from service after engineers discovered a glitch that posed a collision risk. The series has since been retired.

Wiedefeld said there have been no previous issues with doors opening on the 3000-series trains.

Metro plans to eventually replace the 3000-series with the next-generation 8000-series rail cars. The agency has said eventually 85 percent of its fleet will consist of new rail cars.

The 3000-series went into service in the late 1980s.