2004 Prius was front-ended. I did the bodywork and mechanical repairs myself. Did not replace airbags (both fronts deployed), seatbelt retractors (deployed), ABS computer, clock spring yet. Went to start it and in IG-ON mode, car won't go to "P" or any gear. No gear is indicated. Also, on the MFD, the airbag light, check engine, brake warning, ABS, and skid control lights stay lit. Did replace the 12v battery. Question: Which of these can prevent shifting, and the engine from starting in IG-ON mode? Will stored codes from the crash prevent shifting?
Will a dead HV battery prevent starting or shifting, even with a fresh AUX battery? I would guess so.
It's normal behavior for the gearshift to be disabled in IG-ON mode. You can get it into Neutral in IG-ON, but that's it. Are you sure it's not in ACC mode? I just tried that and the dash doesn't light up at all, although the screen does. Pressing Power again (while in ACC mode) gets to IG-ON where most of the dash indicators light up, including the gear display but not the speedometer. Pressing the brake and then the Power button lights up the speedometer, turns off the warning lights, and lights the READY light. If you don't see READY and the speedometer display, the gearshift isn't going to be enabled. If the brake light circuit and/or switch is damaged, then it will not be possible to get to READY mode. The traction battery may be too low to start the engine. If that's the case then a trip to the dealer is the only option short of building your own charger.