Often when a new crank arm has been installed, the magnet for the speed sensor is discarded with the old crank arm. You will need to remove the magnet from the old crank arm (located behind the pedal arm shaft of the crank arm, see reference points A and B) and install onto the new crank arm. The magnet is found on the user's right crank arm for Versions 1 and 2 and on the user's left crank arm for version 3.
Without the magnet, the speed sensor is not able to detect that the unit is in motion. The display may prompt the user to "keep pedaling" or may even shut off after a few seconds (commonly 7 seconds). You may even experience that the cross ramp does not repsond and there are no rpms being shown on the display.
Be advised the speed sensor is pole sensitive to the magnet. If the magnet is not installed with the south pole facing the speed sensor, the unit is unable to pick up motion of the crank arm. You will need to reinstall the magnet with the south pole facing the sensor. You should have spacing of 1/8" - 1/4" between the speed sensor and magnet.