The self-propelled gear set or motor assembly is likely damaged.
- Inspect the self-propelled unit (part number 2826067001).
- Check the motor & gear assembly (part number 2826577002).
- Examine the gear set (part number 2826428002) for stripped teeth.
- Verify the hexagon socket screws (part number 5620157006, 10 required) securing the unit are tight.
If that didn't work, the issue may be internal to the DC motor (part number 2730252001) within the power unit.
Heads up: Do not continue to use the mower with a grinding drive. It will cause further damage to the gear components.