On my 2012 Prius II I was able to access a diagnostic screen in ACC mode by turning the headlights on/off 4 times while depressing the "car" button next to mfd screen. A couple of screens down from the main screen brings up a "signals" screen (similar to that on my 2007 prius), which shows the 12V battery voltage. Doing this while holding down the "settings" button brings up a different diagnostic screen which I didn't explore.
Hi - I'm trying to access the diagnostics screen on my 2010; there's no "car" button, though. Don't guess you'd know how to get there on a 2010? Thanks!
