Responding to a question at her quarterly news conference, Yellen called Wells Fargo's actions "egregious and unacceptable."
The bank has been under fire for the past year after it admitted that thousands of employees enrolled customers in programs without their knowledge or consent. The workers were attempting to meet aggressive sales goals that have since been abandoned. Wells Fargo has admitted that about 3.5 million accounts were affected.
The Fed chair herself has come under some fire, with Sen. Elizabeth Warren, D-Mass., calling for Yellen to dismiss all the Wells Fargo board members in place when the scandal took place.
"We take our supervision responsibilities very seriously," Yellen said. "We are attempting to understand what the root causes of some of those problems are and to address them."
However, she stopped short of saying what regulatory actions the Fed might take.
"I'm not able to discuss confidential supervisory information, and not yet able to tell you what actions we may take," she said, promising only "to make sure that the right set of controls are in place" at Wells Fargo.
The bank paid a $185 million settlement last year with multiple authorities, then added another $142 million earlier this year to settle a class-action suit. In addition, Wells Fargo has admitted that customers were enrolled in auto insurance programs that they didn't need.
Wells Fargo said in response to Yellen's comments that its "Board and management team have taken many actions in response to retail sales practices issues, including changes in senior leadership, eliminating product sales goals for retail bankers as well as numerous steps to make things right with any customer affected by unacceptable sales practices. That work - including a $142 million class action settlement – continues and remains a core part of our efforts to build a better Wells Fargo for the future."