Need help with a diagnosis! 2008 Prius. Transmission issues?

That’s brilliant, thank-you!

 

Update:

The P0A84 code came back up when this other mechanic checked it with a more comprehensive reader, so Toyota had definitely not addressed it after all. We opened up the back, and sure enough there were obvious signs of corrosion (even on the outside of the connector); and clear signs of running & dripping water in the whole area. The fan hadn't been cleaned, so he also gave that a basic clean with the time he had and checked that it was working.

He also replaced the inverter pump, and there's a clear difference: it vibrates more strongly than it was, I can hear the fan spinning, and... the coolant moves!

So you guys were bang on about all of that. Thank-you for the suggestions!

He also checked and cleaned the mass airflow sensor, which definitely hadn't been cleaned by Toyota but apparently didn't look too bad.
Unfortunately when I took it for a test drive, after about 30min, the warning lights came back on...
BUT, even though I was being really heavy handed on the accelerator on hills to see if I could trigger the symptoms, I couldn't get them to show, and the car has much more power than it has in a long time. I may just not have gone long enough for the symptoms to appear, but I'd have expected some sign of them given the way I was driving...

The mechanic had kindly come to my place to check it out so we didn't have a proper contact cleaner - he's going to come back next week and give cleaning the corroded connector another go, or possibly try one of the other solutions suggested on here by @dolj. While he does that I might try give the fan a more detailed clean as well. Fingers crossed that does it... !

 

Turns out they did not