Circuit Description
WARNING: This page is about a different car, the 2003 Lexus LS 430. However, it is still accessible from the selected car via links, so may be relevant.
The telephone ECU (Mayday ECU) monitors the backup battery voltage at terminal SUB BATT. When the voltage drops, this DTC is output. If the battery is normal and recharged with the ignition switch ON, the DTC will be cleared.