There is a truism in Washington that was confirmed last week in Congress: Even less popular than government regulation is a regulator suspected of not doing its job.

Not for the first time, the National Highway Traffic Safety Administration — or N.H.T.S.A. (pronounced NITZ-ah) — was forced to answer for failing to protect consumers. In this case, the failure involved a defective General Motors ignition switch implicated in 13 deaths. While G.M.’s new chief executive, Mary T. Barra, took most of the heat in two days of House and Senate hearings last week, she shared the grill with the safety agency’s acting administrator, David J. Friedman.

Critics, and not just in Congress, have noted that it was not the N.H.T.S.A. that exposed G.M.’s safety lapse and forced the automaker’s recent recalls of nearly 2.6 million vehicles. The defect was discovered by a lawyer and engineer involved in a lawsuit filed against G.M. by the parents of a Georgia woman killed in 2010. Subsequent press reports spurred the recall. Further stoking concerns, the agency twice considered and decided against opening a formal investigation of the suspected defect.

Given that backdrop, Mr. Friedman’s testimony that his agency would have acted differently had G.M. not withheld information about the flawed part won little sympathy from Congress.