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That might be one of several problems: the motor controller board for the belt motor could be defective, controlling wrongly, or the console could be semi-inoperative where speed control is concerned. Also, a button on the console could be stuck down, if 6th speed is the maximum speed setting: that would be the 'speed increase' button on the console in that case.
But 'SE' means speed error, most probably. And that is likely to be the motor controller board for the belt, or just possibly the speed sensor.
Would you like to do work on this treadmill yourself, or would you prefer to hire out the repair work?
I'm glad to hear that you feel up to the task yourself.
The magnetic safety key, when absent or when not working, should not permit the belt to move at all. That's what it's for.
Is the magnet black wheel or the silver button on the wheel?
I'm not sure what you mean. Please repeat in other words, or explain a bit more.
I have removed the panel to check the connections. That sometimes helped.
Do you mean the control panel, and its connections?
Did you do the reconnecting of the control panel connections, while it was On? that can be a bad idea, by the way, if so. But if so, you undoubtedly stirred up the circuitry something considerable, at least.
Yes, the magnet on the wheel will have to pass very close to the sensor with the wire, as the wheel inside rotates, in order for the speed sensor to trigger and work correctly. Ideally the thickness of a credit card should separate them, at closest.
they do cross about 50 %.
That's 50 % of what, please?
The sensor should be exactly opposite the magnet on the wheel, when the wheel has turned to the point where they are closest together.
Loosen the mounting screws on the speed sensor, so that you can move it about. Then move it to where it is closest / most aligned with the magnet when the magnet is closest, and then tighten the mounting screws on the speed sensor to fix it in place that way.
That should fix the problem, hopefully.
Then it isn't alignment.
It might still be a defective speed sensor. Measuring across it, while rotating the magnet to near it and away from it, with a voltmeter, should show a drop in resistance (the switch in the sensor closing) whenever the magnet is near it.