Grassley's contention is that because neither of the two independent investigations into the issue produced a definitive cause or explanation, further digging is necessary. Most of the unintended acceleration incidents in the NASA and National Academy of Sciences reports not attributed to trapped floormats or other problems with accelerator pedals were blamed on driver error, according to the report, but both studies concluded that other unknown issues could be at play.
According to Automotive News, Grassley is particularly concerned about the "tin whiskers" phenomenon, in which tiny threads of conductive crystal can grow on circuit boards.
Toyota has responded to the Grassley letter, issuing a statement that reads, in part, "There is no problem with the electronic throttle control systems in Toyota vehicles – and all the scientific evidence confirms it. So-called 'tin whiskers' are not a new phenomenon and do not represent a mysterious or undetectable problem in a vehicle's electronics."