Traction control problem after new springs

I recently had a weird problem on our 2006 Prius that I thought I might share just in case anyone else ever has a similar problem. We had new springs installed all the way around after breaking a rear spring. The work was done at the local Toyota dealer. Driving home I noticed an odd discontinuity when braking to a stop, very similar to when you hit a bump or ice during regen. I tried to brush it off as just a new feeling to the suspension, but I was concerned. Braking harder, at speed, set off the traction control light. Later I got an ABS error. Something was wrong.

I took the Prius back to the dealer and they said that debris had gotten into the wheel rotation sensor. They cleaned that and all was good. Driving home it wasn't good. I was convinced that they had broken a wire to a rotation sensor or something like that.

It took multiple trips back to the dealer, but they finally realized that a tone ring had been cracked while replacing a spring. It wasn't the rotation sensor, it was the toothed ring that the sensor sensed. Replacing that fixed the problem and our 2006 is back on the road.