A missing safety key, failed console display, wiring failure, faulty motor control board or bad drive motor will prevent the treadmill from starting even though the console display powers up.
Insert the safety key properly when starting the treadmill. If the treadmill won't start with the safety key inserted, unplug the treadmill and check the wire harness connections between the console display and the motor control board. Reconnect any loose wires and repair any broken wires.
If the wiring between the console display and the motor control board is okay, then you may need to replace the console display if it's not detecting the safety key. A technician will typically need to conduct live voltage checks to determine whether the console detects the safety key.
If the console display detects the safety key but the motor control board isn't activating the drive motor to start the treadmill, then you'll need to replace the motor control board. A service technician will typically need to check voltage going to the drive motor to determine whether the motor control board is working.
If the motor control board activates the drive motor but the drive motor doesn't run, then you'll need to replace the drive motor. If you hear the drive motor buzz but it doesn't run, replace the drive motor.