Whether upright or recumbent, most exercise bikes today are magnetic resistance. Magnetic resistance offers the dual qualities of reliability and quiet operation. Another advantage is the programmability of magnetic resistance exercise bikes. Couple this with the reduced knee strain of a recumbent bike, and you have a very efficient means for good cardiovascular exercise. With more electronics than a belt-driven resistance mechanism, repair of a recumbent magnetic exercise bike can be rather expensive, but it is also fixable on your own.
Loosen and remove the bolts that secure the chain guard in place using an Allen wrench or Phillips screwdriver. Remove the chain guard, and set it aside. Rotate the flywheel until the hub sensor battery pack is visible.
Inspect the wire leads at their connection point on the unit. If the leads are intact, with no burn marks, you do not need to replace the unit. Remove the cap from the battery pack with a screwdriver. Replace the batteries, then reattach the battery pack cap.
Locate the speed sensor unit. It is usually near the pedal sprocket. Use a hex wrench to unscrew the nut that holds the speed sensor in place. Pull the speed sensor out just enough to gain access to the battery cap inside the unit.
Remove the battery cover. Insert a new battery or batteries, and replace the battery cover. Reinstall the speed sensor, and reattach the bolt that secures it. Reattach the chain guard, and tighten the bolts firmly to hold it in place.