At a June 18 House subcommittee hearing, Barra said, "I, personally, don't find Mr. DeGiorgio credible."
Valukas also will testify, along with Kenneth Feinberg, the lawyer GM hired to administer a compensation fund to for victims who were injured or families whose loved ones died in accidents caused by the ignition switches malfunction. Feinberg will start accepting applications for settlements Aug. 1.
Read MoreDocuments show General Motors kept silent on fatal crashes
In his report, Valukas said GM lawyers approved a $5-million settlement with one victim without notifying Millikin, which was consistent with corporate policy. Millikin only needed to approve settlements of more than $5 million. In written remarks for the Senate subcommittee, Millikin said that's been changed. He now will be part of the process when a settlement involves a death or serious injury.
Valukas found that Millikin didn't know about pending lawsuits over the defect in models such as the Chevrolet Cobalt and Saturn Ion until a week before the company issued a recall on Feb. 13. Complaints tied to the ignition switch happened as early as 2004.
Barra fired several lawyers on Millikin's staff, including one of his top associates, Bill Kemp.
— By Nathan Borney and James R. Healey, Detroit Free Press, USA Today