The self-propelled unit's motor and gear assembly or drive belt is the likely cause.
Diagnose in this order:
- Check the wheel assemblies (part number 10, 37). Ensure they spin freely and are not obstructed by debris.
- Inspect the self-propelled unit (part number 203). Verify all 10 hexagon socket screws (part number 203-6) are tight.
- Examine the motor & gear assembly (part number 203-2) for visible damage. Listen for a grinding noise indicating gear wear.
- Check the gear set (part number 203-3) inside the unit for stripped teeth.
If that didn't work, the issue may be in the control handle set (part number 202-404). The switch actuator (part number 202-13, 202-21) that engages the drive may be faulty.
Heads up: Do not attempt to lubricate the gear set. Replace worn components.