But the plaintiffs, who could gain from suing the deep-pocketed automakers alongside Takata, argue that the automakers were more deeply involved in the handling of the defect. The fines and costs associated with the scandal have also taken a heavy financial toll on Takata, and it has been searching for a financial lifeline — possibly in the form of a white knight that would effectively take it over.

One of the plaintiffs’ lawyers, Kevin R. Dean, filed an objection to Takata’s plea deal on Monday in Detroit, arguing that the automakers were accomplices in the cover-up. He urged the judge to reject the agreement and for the Justice Department to further investigate the automakers’ role.

The plaintiffs have taken particular issue with the amount set aside for victims in Takata’s plea — a total of $125 million. In contrast, the automakers will have recourse to draw on an $850 million fund to offset continuing recall costs.

Judge George Caram Steeh dismissed Mr. Dean’s objections, saying that Takata’s plea deal was in the best interest of the victims. He said any further action against the automakers should be pursued in civil court, and approved the plea deal as is.

Randi Johnston, 26, of Farmington, Utah — who was injured in September 2015 when the airbag in her 2003 Honda Civic ruptured and metal shards struck her throat — attended the hearing and said afterward that she was shocked by the judge’s decision. The shards severed most of her vocal cords, leaving her able to speak only in a whisper.

“I really don’t have any words right now,” said Ms. Johnston, a plaintiff in the Florida class-action case.