Lerner refused to testify under subpoena in May 2013 about her role in allegedly singling out conservative groups seeking tax-exempt status, after the Treasury Department's inspector general found numerous groups with "Tea Party" or "patriot" in their names subject to special review.
Speaking on "Fox News Sunday," Issa, chairman of the House Committee on Oversight and Government Reform, said Lerner's lawyers informed him she would testify before his committee Wednesday, after saying last week she would not testify.
He added Lerner was not offered immunity.
"We know what she did, in most cases. What we don't know is why she did it. This is someone who came from the Federal Election Commission. She came with a bias towards groups having to disclose [information in order to obtain tax-exempt status]," Issa said.
"There have to be checks and balances, and if it's the IRS, you'd better believe the American people, from the ACLU to the NRA, across the [political] spectrum, want to make sure that politics stay out of the IRS," Issa added.