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.
God bless you