Toyota 2013 Prius Owner's Manual page 547

5-1. Essential information
Emergency flashers
The emergency flashers are used to warn other drivers when the
vehicle has to be stopped in the road due to a breakdown, etc.
Emergency flashers
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If the emergency flashers are used for a long time while the hybrid system is
not operating (while the "READY" indicator is not illuminated), the 12-volt
battery may discharge.
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PRIUS PHV_OM_OM47787U_(U)
Press the switch.
All the turn signal lights will
flash.
To turn them off, press the
switch once again.