Circuit Description - DTC P3120 - Information Codes 257, 259: Notes
WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.
The HV ECU checks the generator temperature and controls the load limitation in order to prevent the generator from overheating. Also, it detects the abnormality of the line connection of the generator temperature sensor and the malfunction of the sensor itself.