Toyota prius Owner's Manual page 286

2-3. Operating the lights and windshield wipers
 The windshield wipers and washer can be operated when
The "POWER" switch is in ON mode.
 Raindrop sensor (vehicles with rain-sensing windshield wipers)
 If the wiper switch is turned to the "AUTO" position while the "POWER"
switch is in ON mode, the wiper will operate once to show that "AUTO"
mode is activated.
 If the wiper sensitivity is adjusted to higher, the wiper may operate once
to indicate the change of sensitivity.
If the temperature of the raindrop sensor is 90 °C (194 °F) or higher,

-10 °C (14 °F) or lower, the automatic operation may not occur. In this
case, operate the wipers in any mode other than "AUTO".
 If no windshield washer fluid sprays
Check that the washer nozzles are not blocked if there is washer fluid in the
windshield washer fluid reservoir.
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 The raindrop sensor judges the amount
of raindrops.
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