Can I start and move in EV mode?

A few more days and sample points have narrowed this temperature window. It worked today at 69F and 73F. It failed yesterday at 66F, though I forget to turn the climate control completely off beforehand, which might be a complication.

68F (20C) would seem to be a sensible breakpoint.

I'll keep trying to narrow this window. Hopefully I don't run into a case of overlapping success/failure temperatures.

 

With nearly two dozen recorded tries at various temperatures, a 68F breakpoint has been a reliable indicator. When ScanGauge reads above that at startup, my Prius goes into EV, below that, it doesn't.

Tested temperatures ranged from 58 to 94, but the great majority of samples were in the 60s and 70s. No samples captured yet at 67 or 68, but I have multiple samples at 66 and 69. Headlights on/off and climate control on/off (heat low when on) have not mattered. ECO mode in all but two cases.

If it can go into EV below 68F, I'm missing some other essential setting.

The ScanGauge temperature is noted after the EV switch attempt, as it displays the final reading from the previous trip while waiting for the first startup measurement.

 

I bet after studying Gen II early battery failures, Toyota found that EV mode overuse was a big factor. Now they went a bit overboard in restricting it for the Gen III!

 

I'm past 50 trials now, and the 68F / 20C rule for EV has not failed.

I haven't observed 68 itself yet, but the window of uncertainty has been narrowed by several starts at 67.

Additionally, on one cold start where EV was rejected, I let the engine warm a bit then shut it off. After a brief wait, a restart produced 69F and a successful EV entry. But circulating coolant or dissipating heat soon dropped the temperature to 67, automatically canceling EV and starting the engine.