ECM Power Source Circuit [08/2020 - ]: Description
WARNING: This page is about a different car, the 2021 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
When the engine switch is turned on (IG), the battery voltage is applied to IGSW of the ECM. When the transistor in the MREL circuit operates, current flows from the battery to ground through the drive circuit of the EFI-MAIN NO. 1 relay, thus operating the relay which supplies power to the +B and +B2 terminals of the ECM.