Lawn mower drive belt. Width: 3/8" Length: 35". If the drive belt is broken, the lawn mower's self-propelling feature won't work.
Lawn mower wheel drive cable. If the wheels do not engage properly then the cable may be broken.
Lawn mower transmission assembly for rear wheel drive models. If the self-propel drive wheels do not engage properly the transmission may be defective. Before replacing the transmission ensure the belt is applying the proper tension to the transmission pulley.