Blind Spot Monitor Fault BSM

The ultrasonic sensors are on the left and right sides of the rear bumper, shown as item ⑥ in parts catalog Figure 84-17, Clearance & Back Sonar, and in the illustration on page 326 of the Owner’s Manual (PDF).

 

Ultrasonics are highly unreliable in high wind speeds (e.g. travelling at speed), and probably don't have the range to cover the distance required. I think the Blind Spot Monitor uses radar modules behind the corners of the rear bumper...

Supporting source: 2016 Prius - the next generation of Toyota's technology ambassador - Toyota

 

That’s a good point. The radar sensors, at about the same locations on the sides of the rear bumper, are shown with Part Name Code 88172 in the second image for parts catalog Figure 84-01, Switch & Relay & Computer.

 

Yes they are radars located in each side rear quarter panels. I'm pretty sure the radars are sealed well against the panels on the inside surface, so unless the outside surface of the panels is covered with a lot of mud or dust, thats probably not the problem.

 

I believe the handbook advises keeping the rear quarter panels clean at all times for them to work consistently well.