Where is the sensor for reading speed and mph

The speed sensor is located on the frame next to the front roller pulley. It will be clamped to the frame on the left side of the treadmill under the motor cover.
The pulley has a small round magnet mounted to it. It may have come loose and attached itself to the metal frame of the treadmill. Check for the magnet, and position the speed sensor as close to the pulley as possible without making contact.
If the speed still doesn't work, replace the speed sensor.