Where is the speed sensor located in the 2003 Sienna?

The speed sensor in a 2003 Toyota Sienna is located at the transmission output shaft. This sensor is used to measure the rotational speed of this shaft. That information is then sent to Your vehicle's ECU where it does the math and gives You a speed on Your dashboard to let You know how fast You are going. If Your speedometer is not displaying any speeds I recommend that You check this sensor first. If the sensor has any loose wires or any damage to it then it will not be able to display the speed properly. Usually You will also get a check engine light at Your dash when the vehicle loses communication with the sensor.