Toyota Prius Owner's Manual page 520

5-2. Steps to take in an emergency
Other displayed messages
Carrying out the corresponding action turns the message off.
Displayed message
(Flashes 3
times)
520
PRIUS_OM_OM47A19E_(EE)
Details
Maintenance of the hybrid sys-
tem (traction battery) cooling
component is required
The filter may be clogged, or
the air vent or duct may be
blocked.
EV drive mode cannot be
switched to.
A buzzer also sounds.
The reason why EV drive
mode cannot be used is
shown (may not be shown in
some instances.)
EV drive mode has been auto-
matically canceled.
A buzzer also sounds.
The reason why EV drive
mode cannot be used is
shown (may not be shown in
some instances.)
Correction
procedure
Have maintenance
performed on the
hybrid system
(traction battery)
cooling component
at any authorized
Toyota dealer or
repairer, or another
duly qualified and
equipped profes-
sional.
Use the EV drive
mode when it
becomes available.
Drive normally for a
short time.