Thank you for your reply, but I was hoping for more detailed info. For example, what is that Detailed Freeze DTC of 1063 associated with the C1300 code freeze data? I was hoping that would provide more information on the detected malfunction. It appears to be intermittent, as the light could go off and on while driving. I could provide the freeze frame data if it helps.
What do you recommend as the easiest way to change the ECU? The service manual says to remove the steering column, but that seems like an overkill to me. Can it be changed but just removing the lower front of the dash? As far as the ebay part, they are being sold by salvage yards, and they offer the same warranty, so I don't see any risk. I was planning on buying the newer part (07-09) as I am wary that a 2005 part could suffer the same problem.
I am somewhat resigned to having to swap the ECU, but being an engineer by training, I was hoping to hone in on the exact problem. The last thing I am going to check is the Power Supply module next to the aux battery. Maybe the connector has an intermittent issue. It's a long shot, but for completeness, I feel the need to check it. At this point, it seems to point to bad/intermittent ECU sensors monitoring the IG voltage, as everything else appears to be functioning correctly.
The only other DTC after a Health Check was an A/C related B1421, which is a Solar Sensor Passenger Side, which is strange as there doesn't seem to be a passenger side sensor. Even in live data, I am unable to see a passenger sensor, only a driver sensor. I am not bothered by this though.
With respect to swapping the ECU, I will try that. Hopefully that provides enough access to remove the ECU. The procedure says to perform initialization on the linear solenoid valve and Yaw rate sensor after swapping ECU. My Prius doesn't seem to have VSC (no indicator), ony standard slip control. I presume I should still perform the yaw sensor initialization?
Just for the benefit for others who might be experiencing the same C1300 code, I have a bit of an update. As part of the process of swapping out the Skid Control ECU, you are required to perform a Linear Solenoid calibration. I thought I would familiarize myself with the procedure before I did the swap, so did the calibration with the existing ECU. Well, after I did the calibration, the yellow brake (!) light has stayed off. It's been two days and the light has not turned on at all. Prior to the calibration, it stays on more than it stays off. The C1300 DTC can still occur and be stored, but it doesn't turn on the brake light anymore. The re-calibration appears to have done enough to turn off the light, but not enough to turn off the DTC. I am going to leave it for a week to see if the light comes on at all, but since I already have the replacement ECU, I plan on swapping out the ECU next week, just to get rid of that C1300 DTC.
After the calibration, the light pretty much stayed off for months, but last week, it came back on and I finally swapped out the ECU. For those so inclined to swap it out yourself, there is no need to drop or remove the steering column. The bottom two bolts are easily accessible but the top bolt is difficult. In the end, I managed to get to it with a long set of wobble extensions shafts, from the front of the dashboard. You do have to pop out the side vents and the panel above the steering column, but that is a whole lot easier than removing or dropping the steering column.
After the swap and re-cal, all trouble codes are clear and the light is off. Hopefully, that's the end of that!