The part(s) or condition(s) listed below for the symptom Lawn mower blades won't engage are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The PTO (Power Take Off) clutch provides a means of manually disconnecting the engine from the blades. When the clutch solenoid is energized, the clutch engages the drive belt to drive the rotation of the lawn mower blades. If the PTO clutch is not getting power, if the clutch solenoid is defective, or if the clutch is worn out, the lawn mower blades will not engage. The PTO clutch is not repairable—if the clutch is defective, you must replace it.
The PTO switch provides power to the PTO clutch. If the switch is defective, it may not supply power to the PTO clutch. As a result, the clutch won’t engage, and the lawn mower blades won’t rotate. To determine if the switch is defective, use a multimeter to test it for continuity. If the PTO switch is defective, replace it.