DIAGNOSIS The handle locking mechanisms are loose. The MJ401E uses two knobs that secure the handle into the upright position.
FIX Lower the handle to the transport position. Inspect the two large plastic knobs. Turn them clockwise to tighten them fully onto the threaded posts. Ensure the handle's mounting brackets are fully seated into the slots on the mower body before tightening. The handle should now be rigid.
IF THAT DIDN'T WORK The plastic threads on the knobs or the metal posts can strip. If the knobs spin freely without tightening, the knobs need replacement. Contact Sun Joe for part number MJ401E-xx (handle knob).
HEADS UP Check and re-tighten the handle knobs periodically as vibration from use can loosen them.