husqvarna · Automower Technical Manual
An object is preventing a drive wheel from turning, or the wheel motor itself is faulty. This can also indicate an issue with the control box.
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Remove any object jamming the Automower.
Check the drive wheel for grass or other objects wrapped around it and remove them.
Test the wheel motors in 'Tools - Test - Power & Motors – Wheel motors' (shortcut 5-2-1-1). Check free-wheeling at 50% and 100% force, and block wheels to check for 0 cm/second speed.
Rotate the wheel by hand to check resistance, then disconnect the motor cabling and rotate again. If it is easier to rotate now, the control box may be faulty. Replace the motor if defective.