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For anyone else to find this thread in the future. It was indeed caused by a very weak 12v battery. The car is still in service with almost 250k miles no further repeats with the new battery.

Update for anyone interested or having similar issues: Ive had one more "incident" in the past few hundred miles, besides that no other symptoms or way to reproduce it. My 12v battery was extremely weak, it would die if i leave the lights on for more than a minute or two, and ive replaced that today as a precaution if that was somehow confusing the transmission. Prius forums indicate weak 12v batteries can lead to much electrical weirdness. The position sensor is also referred to as a resolver, Prius forums indicate the only time they have an issue with this sensor is if the wiring is damaged somehow such as by road debris , Prius resolver wiring is external to the transmission, while ours is internal so wiring damage seems almost impossible on this transmission. I was also able to confirm that PATS does tie into the transmission, so if i do end up replacing it down the line that will likely require reprogramming the PATS system. As things stand, ill keep driving it with the new 12v battery and see if i run into this issue again.

Hi Everyone, Im having an interesting issue with my transmission on a 2010 Milan Hybrid with 190k miles. Seemingly the Generator speed sensor in the transmission is starting to fail. Googling and research does not show any documented failures of internal transmission failures on 2010-2012 hybrids so I wanted to see if anyone had seen anything like this before and if they havent i can document my experience. Symptoms so far are as as follows: 1st incident - Cruising at about 40mph with mild acceleration (not in ev mode), engine suddenly revved up as if it was a manual transmission and i was in neutral, I immediately released the gas pedal since im used to driving manual and it was like missing a gear shift. No lights, car continued to drive normally, i did not even stop, reapplying acceleration worked normally. Checked codes with forscan without turning car off, nothing relevant. 2nd incident a few weeks later- Similar speed conditions, got the SSN message and lost all acceleration power, not even ev acceleration. Coasted into a parking lot and pulled codes, got a "Generator Speed sensor performance" code, i forgot the exact code since that was a few weeks ago, i think it was P0A4C. Restart of the car and it drove normally. 3rd incident- 15-20mph, got the engine revving, sounded pretty close to 5-6k rpm, release and reapply gas pedal and car drove normally. Checking the codes later I pulled P0A50. At this point its pretty obvious there is some intermittent issue with the generator positioning/speed sensing, this makes sense as torque from the engine goes through the generator motor in the transmission to the wheels, if that motor does not properly apply torque, the engine will not be able to send power to the wheels and will just rev. Other interesting notes: I have another 2010 with 140k miles, it operates much quieter in ev/regen, there is much less whine from the transmission but this one has sounded more or less the same over the miles ive had it. Trans fluid was changed at 130k when i got the car and at 185k when i replaced an axle seal due to a minor leak. When i got the car, it was an auction car because it wouldnt start when hot, that turned out to be a failed temperature sensor in the HVB, interestingly there was no codes of any kind, but one of the multiple temperature sensors in the pack was reporting -40degrees. I believe this vehicle has been in atleast one wreck over its life, and looking at the engine bay there are signs of the engine previously being taken out(maybe they tried a new engine/transmission due to the no start condition) There is not even any mentions of these codes in the service manual. At this point the solution seems simple, used transmissions are about 300$ and i can do the swap myself in a weekend, although it seems like the PATS system may need to be reset since supposedly PATS ties into the transmission not the PCM on the hybrids? Ive got Ford IDS access so that should be no problem if needed. Anyone seen transmission failures of any kind in this gen fusion hybrids? Does anyone know if PATS ties into the trans or PCM? The 2013+ guys seem to be swapping transmissions without PATS issues.

Joined the forum just to comment on this, I have 2 2010s with this issue and a 2009 escape with i think what is the same issue. Its crazy to think that every single 2010 fusion hybrid will have this issue this year. Just in case anyone else is wondering, you can reset the age yourself using software Forscan (you can do free trial, but i would encourage donating and supporting the developers) and a suitable obd adapter. From my experimentation the age is held in the "As-Built" data of the BECM, originally the discharge power limit is 27000watts temperature permitting, once the Age in the BECM hits 9 years, that goes down to 14700, and am assuming will go down even more as the age goes up. (i believe it stops at 15 years and what happens then is anyones guess) Im assuming this was a cover your a** move by the engineers, a 10 year old battery capable of 100 amps at 300 volts could cause a very large fireworks show if something went wrong, so they limit it to reduce the stress on it as it ages. So reset the age at your own risk. But my personal opinion is that this will shorten the age of the battery in the long run, when its limited in software it is under less stress so will obviously last longer, but thats irrelevant since by the time the battery truly fails the rest of the car will be ready for the junkyard and the small mpg difference is worth it. I expect the battery to actually last 13-15 years before true failure. safety wise, i think the battery should be plenty safe even as it ages as we have seen with other NIMH hybrids. If anyone in Nashville needs this done to their fusion, please feel free to reach out!