Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The sensor should be only a credit-card's thickness away from the magnet it passes close to. Or closer, at the closest.
What error exactly is the 'low speed' error you have seen, literally? it might possibly be interpreted differently.
If you have (and know how to use) a voltmeter, try a 'power Off' test of the sensor switch, unplugged from everything else, with the 'mill parts rotating but with no power to the treadmill.
Put the speed sensor's two leads, on the plug or terminal unplugged from the other stuff in the treadmill... across those two leads... and measuring resistance through the sensor switch, the switch should close (0 Resistance as opposed to infinitely large resistance) at every pass by the magnet, and if it doesn't, then the switch (or perhaps the magnet? examine it too) is defective and needs replacing - if it truly passes close to that magnet.