Toyota 2012 Prius Owner's Manual page 284

2-1. Driving procedures
Horn
■
After adjusting the steering wheel
Make sure that the steering wheel is securely locked.
The horn may not sound if the steering wheel is not securely locked.
(→P. 165)
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PRIUS PHV_OM_OM47818U_(U)
To sound the horn, press on or
close to the mark.