The inquiry represents the latest backlash against Apple's decision to slow down older iPhones, a move that the company says is created to prevent the devices from abruptly shutting down when older batteries are running low or operating in cold weather.
In fact, the company is now facing an extraordinary number of class-action lawsuits both in the US and overseas.
In countries such as France, "planned obsolescence" is against the law.
Thune, who heads the Commerce, Science and Transportation Committee, wrote to Apple CEO Tim Cook that regardless of intention, the lack of transparency by the company was a reason for his questioning.
Sen. John Thune, R-South Dakota, on Tuesday sent Apple a letter with questions about a software update that helped prevent unexpected battery shutdowns in older iPhone models but reduced their speed, according to multiple media outlets that have read the yet-to-be-released letter. After all, Apple failed to provide any kind of notification to users who had older iPhones that it would be slowing down their devices. He has told the tech giant that they have until January 23 to answer his inquiry.
This led Apple to announce a temporary price cut for iPhone replacement batteries. However, it wasn't entirely clear this power management feature would slow down older models of the iPhone with each update.
When an iPhone's processor makes a large current demand from a device with an ageing battery, this can risk damaging components. Meanwhile, the rest of the devices covered under the program, including the iPhone 6s, 7, 7 Plus and SE, should be eligible for replacement "without extended delays".
First it was Samsung with their Note 7 now apple has a new battery problem that resulted in one of their stores having to be evacuated due to an exploding iPhone battery. The company also will update its iOS operating system so users can see whether the battery is in poor condition and affecting the phone's performance.