The current software for the motor/generator control and hybrid control ECUs can cause high heat to develop around some transistors and damage them. If this happens, lights on the dashboard would warn drivers. However, in some instances the problem could result in a failure of the hybrid system.
Dealers will upgrade the software for both ECUs to fix the issue, and Toyota will notify owners in the US by mail about scheduling repairs. According to Reuters, there are also 340,000 vehicles in Japan and 160,000 in Europe that need this, as well.