GM did change the ignition switch, in 2006, but the process did not comply with the company's own rules, documents and testimony show.
Parts maker Delphi told congressional investigators last week that the redesigned switch on the 2007 models was harder to move out of position, but the force required to turn the switch was "still below GM's original specifications," according to a timeline released by the investigators.
The document showed that the part number did not change, when redesigned, an issue which GM said hampered its own internal investigation.
"It is inconceivable," Barra said, when asked about the design change without a corresponding change in part number. "It is not our process."
Moreover, several fields on the document marked as "required" are left blank or with "N/A", including purchase order number and "validation engineer".
The document has the signature of GM "lead engineer" Ray DeGiorgio, who could not be reached for comment. In a 2013 deposition in a suit against GM, DeGiorgio had said he was unaware of a change in the part.
A retired GM manager familiar with the automaker's engineering and manufacturing procedures said that another manager would have had to sign off on the part, by company policy, given DeGiorgio's relatively low seniority. "So who approved a design change without a part number change?"
Barra confirmed that DeGiorgio is still employed by the company and said she has not yet heard his explanation for signing the 2006 document.
The redesigned ignition switch was installed on 2007 Chevrolet Cobalts and Saturn Ions in late 2006, GM has said.