Toyota PRIUS Manual page 368

2-4. Using other driving systems
Radar sensor and grille cover

Always keep the sensor and grille cover clean to ensure that the vehicle-to-
vehicle distance control operates properly. (Some obstructions, such as
snow, ice and plastic objects, cannot be detected by the obstruction sensor.)
Dynamic radar cruise control (vehicle-to-vehicle distance control mode) will
be canceled if dirt is detected. (Constant speed control mode can be used).
 Warning lights, messages and buzzers for dynamic radar cruise con-
trol
Warning lights, warning messages and buzzers are used to indicate a sys-
tem malfunction or to inform the driver of the need for caution while driving.
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PRIUS PHV_OM_OM47831E_(EE)
Grille cover
Radar sensor