DIAGNOSIS An incompatible or counterfeit brush head, or one that isn't fully seated. The iO Series requires genuine Oral-B iO-specific brush heads with the internal metal ring.
FIX Remove the brush head. Look inside—you should see a silver metal ring. Align the head perfectly straight and push it on until you feel a distinct, firm click. The head should not wobble.
IF THAT DIDN'T WORK Try a different, known-genuine iO brush head. If the problem disappears, the first head was defective or fake. If the stuttering continues, the drive mechanism in the handle may be faulty.
HEADS UP Using old Oral-B Genius or other series brush heads will cause this stutter and can damage the handle. Only use packaging marked for iO Series.