In a letter to Apple's Chief Executive Tim Cook, a copy of which was viewed by The WSJ, Thune asked how the company tracked complaints of customers over processing performance and if it explored offering refunds to clients who paid full price for replacing a battery before the company cut replacement cost in December.
The senator requested the company to answer by January 23.
On December 22, US nationals brought a class action lawsuit against Apple aft the IT giant had revealed it slowed down older versions of iPhone to enable them to run longer.
READ MORE: Apple Finally Reveals Why Older iPhones Suddenly Become Slow
Separately, Paris prosecutors announced on Monday that they had opened an investigation into a suspected fraud by the tech giant following a complaint by a consumer organization.