REGEN COMPUTER FAIL DROPS 5 MPG

"Regen computer" isn't the name of any computer in a Gen 2 Prius.

There are about a dozen computers in the car. One is called the skid control ECU, and it is responsible for braking, ABS, and traction and stability control. It can light the three related warning lights on the dash, for brakes, ABS, or skid control. Another computer is called the HV control ECU, and it is responsible for coordinating the gasoline engine and the electric propulsion, and it is the ECU in charge of regen. That computer can light the master warning triangle.

Regen during braking is handled by those two ECUs negotiating with each other; the skid control ECU reports how much braking the driver wants, the HV control ECU responds with how much regen it can do, and the skid control ECU subtracts that and uses the brakes for the rest.

If there are problems reported by either of those ECUs, often they will call off those negotiations, and the skid control ECU will handle all braking using the actual brakes.

But that's something that happens if one of those ECUs is reporting a trouble code (and has warning lights on) right now. It's not something that happens if one of them reported a trouble code in 2017 and isn't reporting it now.

Do you have any warning lights on the dash?

If you have any, then the thing to do would be to find out what trouble codes are being reported now. If you post those here, we can be a lot more help.

If you don't have any, then it's safe to say whatever issue was being reported in 2017 is not being reported now.

 

@Meter guy, the full name of the HV control ECU mentioned is Hybrid Vehicle Control ECU. I make note of this as the initialism HV is more commonly thought of in the context of high-voltage electricity. Changing the perception from 'high voltage' to 'hybrid vehicle' also then changes the nuance to better understand that the HV Control ECU is very much the conductor of the orchestra, or the hybrid vehicle system in this case (a point Chap often points out) and not just focused on the battery with high DC voltage. A lot of people immediately jump to thinking about the hybrid vehicle battery (as Toyota calls it) when they see the initialism HV however, as explained, in this case, HV is much more than high voltage, it is the entire hybrid vehicle system.

I just wanted to point out this in the hope it will help with the understanding of the hybrid vehicle system.

Secondly, I will point you to this thread started by another member who I believe is trying to get a handle on a phenomenon that sounds like it might be similar to what you are experiencing. I do not fully understand all the technicalities of what he is saying, but I have experienced what he describes from time to time. In my case, I didn't do anything and, after some time, the car returned to what I had experienced as normal on its own. I also do not experience the phenomenon very often but when it did do it I did wonder to myself what was going on because the car would not behave as I expected it should do. The thread in the later pages does wander off-topic as various people have tacked on and hijacked the thread. Just skip over those detours so you don't lose focus on the main topic.

I hope that helps.