If your treadmill has started acting funny, the first step to be taken is to troubleshoot the problem yourself. Alot of times, even for small problems, one ends up calling the repairman or the customer services, and ends up paying a bomb for a condition which could have been easily solved by oneself. Here is a few thing you should know about treadmill troubleshooting.
First unplug your treadmill. Wait for a few minutes and then plug it back again in its single outlet surge suppressor (which should be connected to a grounded outlet).
Check for the circuit connection thoroughly. Many a time, there is a chance that the circuit breaker has tripped. This can be checked for by looking at the tabbed end of the circuit breaker (located near the power cord). If it is protruding, wait for about 5 minutes and reset the circuit breaker by pushing the tabbed end back in.
Now, try to remove the treadmill key from its slot and try sliding it back into the slot. Check if the key is properly seated.
Remove the treadmill key from the slot in console. Unplug the treadmill.
Remove the three screws that are holding the hood cover (in front of the walking belt) of the treadmill in place.
If you carefully observe the left side of the treadmill belt, there is a small magnet on the outer side of the pulley (which is located on the left side of the belt). This magnet should ideally line up with a small cylinder (placed in the treadmill frame, which is also called Reed Switch). So when you turn the pulley, the magnet lines up with the Reed switch. Check the length of the gap between the switch and the magnet. It should be about 1/8 inch wide. If it is not, remove the screw that is holding the Reed switch, and try moving it away or towards the magnet to correct the gap until the distance is 1/8 inch wide.
Walking Belt Troubleshooting
Again unplug the treadmill and remove the key from the console slot. Keep your hands below the treadmill belt and try to lift it. If the belt is centered and properly tightened, one can lift both the edges of the belt about 2-3 inches.
Carefully observe the back side of the treadmill frame. You will find two rear roller belts to either side of the treadmill belt. If the belt is held too tightly, rotate each bolt counterclockwise (take a one quarter turn), to loosen up the belt a bit. If the belt was loose, when you had lifted it, then turn the rear bolt clockwise (by 1 quarter turn).
Now rotate the left roller belt counterclockwise (by ½ turn), in case the belt is not exactly in center and has moved towards the right. Similarly, if the belt has moved more towards left, rotate the roller bolt in clockwise direction (by ½ turn).