Sometimes, troubleshooting is a case of spotting user error. If your Horizon treadmill doesn't turn on, you might have forgotten to do something simple like plug it in, turn the power switch on or insert the safety key. But if you've ruled out user error and the treadmill still won't turn on or doesn't work correctly, your Horizon treadmill may have a mechanical malfunction. Some problems, like a slipping treadmill belt, are relatively easy to correct with basic troubleshooting measures.
Remove the safety key from the treadmill console. If it's a magnet-style key, test the magnet against a metal surface to make sure it still works. If it's a slot-style key, check to make sure it triggers the reed switch when inserted, and manually reposition the reed switch trigger if necessary. Reinsert the safety key in the console. If you see all dashes or "safety key off" displayed on the treadmill console before removing the key, this may solve the problem.
Troubleshoot non-functioning console keys by unplugging the treadmill and firmly massaging all console keys with a circular motion. Plug the treadmill back in and check the keys. If they still don't work, unplug the treadmill and wait 60 seconds for all energy to discharge. Then, unscrew the console faceplate from the housing. Gently lift the faceplate off and check that all the thin, clear, white-striped ribbon cables are securely fastened.
Unplug the treadmill. Then, unplug and re-plug the console cable from the treadmill to be sure it's connected firmly. Double-check all other connections to make sure they're secure. This may solve problems with the elevation function not working properly, console keys not working or the running belt not working properly.
Remove the motor cover with the treadmill still unplugged. Wait until all the lights on the circuit board have gone out before proceeding. Check that all wires are securely fastened, including the 12-wire harness on top of the circuit board. This troubleshoots problems with the incline not adjusting properly or the running belt not moving.
Find the rear roller bolts in the rear of the treadmill frame. If the belt slips as you run, tighten each bolt a quarter turn with an Allen wrench, then run the belt to see if it still slips. If the belt slips drastically to the right, tighten the right bolt by a quarter turn. If the belt slips to the left, loosen the right bolt a quarter turn. Then run the treadmill and observe the belt to see if it slips. Continue adjusting the bolts a quarter turn at a time until the belt no longer slips.