VSC, ABS, (!) and emergency brake light on, replaced 12v battery but still brake issues

You need to clear the skid control ECU DTC. Google that or search this forum and you will find some answers. More than likely, someone else will ring in with more detail instructions.

 

1. Check the inverter coolant reservoir for fluid turbulence when the Prius is READY. If you do not see this then the inverter coolant pump has failed and needs to be replaced.

2. Try clearing the skid control ECU DTC by using a jumper wire on pins 4 and 13 of the OBD-II connector. See posts 27 and 29 here:
I'm in it now - Front wheel bearing/hub replacement | Page 2 | PriusChat

and see post #7 here:
How do you reset ABS light & yellow ((!)) ? | PriusChat

3. If the above doesn't help and the braking system remains impaired, then you should immediately seek professional assistance. Or if you want to DIY further, buy Mini VCI (see Amazon.com) so that you can retrieve the DTC. This requires a Windows XP or Windows 7, 32 bit OS laptop. Good luck.

 

You understand correctly.

An alternative to paying the $170 diagnostics fee is to DIY by buying Mini VCI as I suggested above. However, once you have the DTC then you are still faced with the need to take some action. If you would not be comfortable repairing the brake system yourself, then you'd might as well go to the dealer now.

 

The skid control ECU is located near the steering column. Access requires that you remove the steering wheel (including the driver airbag) and the instrument panel plastic trim (including the front passenger airbag).

This is not an activity for the novice. You can obtain factory repair manual info at techinfo.toyota.com if you want to investigate what is involved.

The manual shows the troubleshooting tree used to determine whether the brake stroke sensor is good or bad. The skid control ECU should be replaced only after all other causes have been eliminated, including the sensor and the wiring harness.

For $170 you might think that the troubleshooting tree was followed. If the result of this was not documented on the repair invoice, then my guess is that once the DTC was retrieved, the tech specified the most costly repair.

It is much easier and cheaper to replace the brake stroke sensor, located near the brake pedal assembly. That could be a novice-level project.

For $75 it is quite reasonable to try a used skid control ECU vs. paying four-digits for a new ECU; assuming that you are game to do the needed instrument panel and steering wheel disassembly to get there.

I also think you should clear the DTC now if that was not already done by the tech; and see how long it takes for the DTC to return. You may be able to get a few more months of service out of the car before the fault returns, giving you a chance to thoroughly consider your repair options.

 

If the sensor is cheap I would be inclined to swap it out and hope the problem was fixed.

 

Exactly. $133 plus shipping, here:
2009 Toyota Prius Parts - AutoNation Toyota Gulf Freeway Parts

The price of the skid control ECU is $1,155 plus shipping at that dealer.

 

Prius_Aki, where are you located? Please put that in your profile. There might be an independent shop willing to install a used skid ECU.

 

You have 2 really good hybrid shops around you. Luscious Garage and Art's Automotive. They have the expertise to fix anything on your car.

 

Yes

 

Excellent!

Consider replacing the 12V battery if it might be getting on in age. Random DTCs can result from a weak 12V

 

"Afterwards, they apparently reset the DTC and whala, the braking issue went away (how is that possible?)."
The reason that this is possible is that the fault is intermittent. In post #7, I suggested that you clear the DTC if that had not already been done. The fault will eventually return, and when it does I suggest that you first replace the brake stroke sensor.

I doubt that the fault is related to the 12V battery being weak.

 

I'll take your opinion over mine anytime.

Question though: why would clearing the DTC change the brake performance back to normal ?

 

Because the fault is intermittent. So the brakes will work OK after the DTC is cleared, until the fault returns.

While the DTC remains logged in the skid control ECU, the braking performance will be degraded because Toyota wants the owner to take remedial action regarding the fault, not just ignore it.

Along the same lines, it could be that since the skid control ECU's performance is suspect given the DTC logged, the hybrid vehicle ECU assumes a degraded "limp home" performance profile which results in poor braking perceived by the OP.