Check your owner's manual for diagrams and definitions. You'll need this information when you put the elliptical back together. You'll also want definitions in case you need to call the customer support line for assistance.
Be aware that the resistance is often the part that requires repair. Resistance is managed magnetically, so often if the machine is working, but the resistance isn't setting correctly or at all, check the bolts that secure the magnets. If they are loose, the resistance will slip. If the bolts are fine, check the magnets themselves by using a refrigerator magnet and see if it is attracted or repelled by the your fridge magnet. If not, the magnets have de-magnetized and need to be replaced.
Consider that if the foot pedals are attached with bolts, any slipping can be fixed with a quick use of the wrench. But if the foot pedals are damaged or breaking, order new foot pedals (see Resources below) rather than run the risk of a pedal breaking while you're on the machine.
Check the hand bars. While not all people use them, as they can elevate your heart rate by up to 20 percent, they can become worn, too. While loose hand bars typically need a simple tightening of the bolt, be sure not to overtighten as the bolts can break.
Know that if the axis is bent, it's likely you need a new machine. The axis is the central alignment of the elliptical. If the machine was used by a person who is over the maximum weight limit, it can be bent or damaged. It's not an easy fix and a better idea is to upgrade to a sturdier machine.