Prius Transmission Issues

Sounds rough! I had a similar experience with my old Opel Astra, though the exact symptoms were slightly different. The 'emergency mode' thing and the weird shifting definitely ring a bell. In my case, it turned out to be a faulty sensor related to the transmission, causing it to think the oil was overheating, even when it wasn't. It's like your body thinking it's cold even on a hot day so you start shivering uncontrolably. Did you check the oil level? Maybe it's low, and the sensor is just doing its job by reporting a problem?

Regarding the oil level: would a significantly low gearbox oil level definitively trigger the observed symptoms, or could the temperature sensor itself be malfunctioning irrespective of the oil level? I am trying to differentiate between a genuine low-oil situation and a sensor failure presenting as a low-oil indication.

That's the million-dollar question, isn't it? It's tough to say for sure without proper diagnostics. The low oil could be causing the sensor to freak out, but the sensor could also be lying like a bad politician. Given the symptoms and the sensor error, I'd say it's probably best to get it to a shop. They can properly diagnose whether it's the sensor itself or a genuine low-oil problem that's causing it. Replacing the sensor preemptively might not fix anything if the level is low!

I took it to a workshop, where they confirmed a faulty transmission oil temperature sensor. Further inspection revealed that the low oil level was the culprit. The transmission oil temperature sensor and associated repairs cost approximately 365€. Thanks again for your guidance!