When checking a treadmill that will not run I look at a lot of things beginning with the power supply. I first make sure that the outlet is grounded and that the polarity is correct. I do this with an outlet tester (see images below). Then I make sure that the voltage is correct, which should be within a volt or two of 120 VAC.
The next thing I test is to make sure that the two AC power lines are providing that 120 VAC at the motor control board. In this case it was. The upper console on this treadmill would light up, and even show speed even though the walk belt was not moving.
The next thing I checked was the drive motor. After first making sure that the treadmill is unplugged from the wall, I unplug the two power lines from the motor control board that go to the motor (usually red and black). Using jumper wires I connect these two wires to a 18V DC battery with a good charge on it, like those on a portable electric drill or impact. If it runs the walk belt then you know that the drive motor is good.
I knew at this point that we were dealing with a bad motor control board. I did check the wiring harness going from the upper control board to the lower motor control board to make sure there was no damage and both ends were properly connected. I also tested the leads on the motor control board that go to the motor to just make sure there was no DC power coming out to the motor which the console said that the walk belt was running. There was none. I ordered a new motor control board for this treadmill.
If you are a do-it-yourselfer and my information has helped you, please consider ordering your parts from me. You’ll get the best price of anywhere else online and I will make a little profit in return for providing this free, helpful information to you.