Disconnect the treadmill from any power source to which it is connected. Make sure the treadmill has no power running through it at all. Remove the front covering over the motor of the treadmill to expose the main circuit board and motor assembly.
Disconnect the circuit board wires leading to the motor. In most cases, the wires are black and red. The motor should also be removed from its mount at this time.
Move the motor to slacken the pulley, then lift the motor to get underneath. That is where the treadmills speed sensor will be located. It is usually a square or rectangular shaped green module with connectors at both ends.
Use a finger to check the speed sensor's rigidity in its bracket. The malfunction could be because it needs to be tightened or adjusted in some cases. At this point, do a test. Remove the treadmill speed sensor and use the voltmeter to test the circuit. Check manufacturer specifications on the accurate use of the voltmeter.
Remove the speed sensor completely and replace it if necessary with your new speed sensor. Then re-attach the wires that lead to the circuit board. Return the roller pulley and motor to their original positions and replace the protective cover.
Plug in the treadmill or attach power source. Once on test the treadmill to make sure the speed sensor has been fixed. This can be done by running the treadmill for 15 minutes at low speeds monitoring it carefully to see that it is performing as expected. Check manufacturer specifications to set up your treadmill to desired settings, (refer to owners manual).