Vehicle Signal Check Mode test,13.4v while driving,13.9v on Park

Seems to me that's testing something to do with your 12 volt battery at rest and charging or something You're hybrid battery would have nothing to do with 12 or 13 volts The inverter DC to DC converter dictates what's on the battery from the inverter when the car isn't ready I don't know about the signal check business ready the car and measure across the battery terminals with a voltmeter It should be what you just said it was on 13.9 that's about right 13.4 at rest I'm interested in that rest after about 45 minutes not 2 minutes after the car's been unreadied

 

If you want to test the 12V get a digital multimeter and measure with the positive lead on the jump post (under the red flap in the underhood fuse box) and the negative lead to one of the unpainted bolts on the inverter. The night before release, but do not open, the hood, then turn off the car. The next morning open the hood and measure the voltage. Do not enter the car first - opening the door causes it to pull current from the 12V and it will change the reading (show a lower voltage). Do not measure immediately after stopping the car as the 12V will have a lot of surface charge which will make it look better than it is.

To measure the HV battery voltage you need software and a tool. One option is a Mini-VCI cable and a copy of Techstream, running on PC (either natively in Windows or in Windows in a virtual machine). Another is a good OBD2 reader and software like Dr. Prius which runs on your phone. You can also read and clear trouble codes with this combination.

 

what is the purpose of the vehicle signal check mode test, inverter behavior?

and yes, you will have to replace your hybrid battery soon

 

This tells you exactly nothing about the state of your HV battery.

The only thing it tells you is that your inverter DC/DC converter is putting out enough voltage to keep the 12 V battery charged.

 

Vehicle Signal Check is a diagnostic screen on the MFD, showing you the status of a small handful of vehicle signals that are wired to the MFD for one reason or another. So it shows you whether the taillights are on or off (because that circuit is wired to the MFD so it can alter its brightness), and whether the backup lights are on or off (wired to the MFD so it knows when to show the backup camera), and whether the parking brake is on (I forget why that's wired to the MFD, but it is), and maybe a couple other things like that. It also shows a reading of the 12 V system voltage, taken inside the MFD itself (which usually seems to be a good fraction of a volt lower than you would see with a meter right on the 12 volt battery). The Vehicle Signal Check screen is used by some as a convenient way to check the voltage of the 12 volt system, ok as long as you remember that fractional volt difference.

There isn't anything on that screen that tells you anything about the high-voltage battery.

You might, just as a general prediction based on the age of the car, but nothing in the OP helps make that decision either way.