Turned on my car one day and saw this: “Caution – The Transmission P lock mechanism is abnormal. Park your car at a flat place, and apply the parking brake completely.”

I’m thinking: “It looks pretty flat to me, buddy.”

The 12V battery (not HV battery) is failing. Here’s how to diagnose the problem and repair the issue.

** There’s also way to access a nifty hidden maintenance menu on the MFD (Multi Function Display) – See video at the bottom of this post **

Diagnosis:
Gen 2 Prius bad 12v battery – symptoms

Tutorial:
How to Replace the 12v Auxiliary Battery 2004-2009 Toyota Prius
http://priusdiy.com/tutorials/interior/12vBatteryReplacement.html

Battery:
Optima Yellowtop DS46B24R
Purchased at O’Reilly Auto Parts, price matched with Pepboys for $129.70 (Originally over $220).
This Optima battery is supposedly “exactly the same size as the OEM battery.”

My Video: My First Readings from MFD Maintenance Mode and Scanguage 2 after the 12V battery swap