Treadmills are designed to operate with a safety key. When the key is inserted into the console, it triggers a switch that allows electrical current to flow and activate the treadmill functions. Safety keys can be misplaced or damaged when not stored properly. Attempting to bypass the switch and connecting electrical leads permanently is an unsafe solution to the problem of a lost or damaged key. If your treadmill uses a convex or ribbed key, it might be necessary to install a new safety key component. But it's possible to build a straight safety key for a treadmill by compiling a few items and hand tools.
Make sure the treadmill is plugged in. Use a utility knife to cut a piece of poster board the width of the safety key slot and about 3 inches long. Straddle the running belt and insert the piece fully into the safety key slot and press the "Start" button. If the display panel illuminates you can move on to step 3.
Cut an additional piece of poster board the same size as the first. Double the pieces evenly, insert them into the key slot and press "Start." The display panel should illuminate. Retract the pieces from the slot. The display panel should go off.
Use one of the poster board pieces to mark and cut a piece this size from a uniform portion of a plastic file box. Use a hacksaw to cut the plastic. This piece will be the safety key. Smooth the edges and corners of the key with a flat file.
Drill a 1/4-inch hole at one end of the new key. Insert a length of 3/16-inch diameter braided cord into the hole and tie an overhand knot to prevent the cord from pull through the hole. Cut the braided cord about 36 inches long. Attach the free end of the cord to a plastic spring clip that's used to attach the cord to your clothing as you workout on the treadmill. Conduct a test to make sure the safety key pulls from the slot if you step off the treadmill.