JLG 1500SJ Operation And Safety Manual page 154

SECTION 5 - LCD DISPLAY PANEL - GENERATION 1 & 2
AXLES RETRACTED IN OUT OF TRANSPORT POSITION
When the axles are retracted and the machine is in the out of
transport position, the display will switch between axles
extended and retracted every 0.5 seconds to tell the operator to
extend the axles so the boom can be raised.
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