Today’s self-propelled lawn mowers either have steel or plastic drive gears. The steel gears never seem to wear out but their spring loaded woodruf keys that are on the axle can rest and stick in place. When this happens, one side or both can stop working.
If you have a plastic drive gear on the wheel, it can wear out and stop pulling the drive wheel.
In either case the drive wheels will need to be removed and the drive system inspected. While you are at it, inspect the drive belt for wear. If it is loose or rides deep in the groove, it is time for a replacement.
Carefully inspect the drive gears to make sure they are not missing any teeth or are worn out. Grass clippings can also get into them and fill the grooves between the teeth. Clean them out good and replace any worn parts. Also check the drive shaft on models that use the spring and woodruf key. Clean out the slots and ensure everything is dry and free of grease and dirt.
If this project is more than you want to tackle, visit our Locations page for a self-propelled lawn mower repair shop near you.