Toyota Prius Owner's Manual page 306

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 messages and buzzers for dynamic radar cruise control
Warning messages and buzzers are used to indicate a system malfunction
or to inform the driver of the need for caution while driving.

Certification
The latest "DECLARATION of CONFORMITY" (DoC) is available at the fol-
lowing:
http://www.globaldenso.com/en/products/oem/index.html#2
306
PRIUS_OM_OM47A19E_(EE)
Grille cover
Radar sensor