Be careful when testing. A danger of a fall and serious injury is always present.
E21 means the treadmill is not picking up a speed signal once the start key is pressed. First, determine if the motor is turning before you get the E21 or not. If the motor turns, the speed sensor is the most likely problem. If it does not, you can manually turn the belt with your feet before you get the E21 and you should get a speed reading but eventually you will get an error code because it is almost impossible to keep the speed constant enough for the computer.
Try running it through calibration with the motor cover removed. Most that give an E21 message have either a MC-54, MC-70, MC-2000(H), or MC-2100 controller. If it doesn't run in calibration, make sure the PWM LED is flickering on the power board in the motor area when the duty cycle is set to 85 (the instructions will make this step more clear). If the motor does not run, test the motor with a cordless drill battery. The instructions for testing the motor can be found at the following link. Motor-Testing
If the motor tests good, the wiring is good, and it still won't run in regular operation of the treadmill, the controller is out and you need a new one. In many instances we have upgraded controllers that have fuse protection and other features that prolong the component's life.
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