P0136 Toyota Sienna 2003XLE

Hello, Your O2 sensor circuit, including the O2 sensor needs to be inspected. Yes you should have voltage at all sensors, however, when there is an issue, the ECM can run in open loop, meaning it does not request information from the O2 sensors. Clear the code, do not restart the engine, and test the sensors for voltage. If it is not present, then there may be a wiring issue. Since the B1S2 has voltage, yet it is the cause of the code, you may want to start by checking for proper resistance across the sensor.