I have a 2008 Prius that I'm taking from Florida to Minnesota. I drove it around in Florida for a few days and everything ran great, but about 4 hours into the trip back to Minnesota, the red ! triangle light came on with a message on the display screen. It said "There is a problem with the transmission "P" lock mechanism Park your your car on a flat surface and fully apply the parking brake." This happened as I was driving, so I pulled over and did what it told me. After shutting down the car, I tried starting it back up and none of the gears worked. P, R, D, or B. The message still displayed and the red ! light was still on. After shutting it on off several times, it finally started. No warnings. I found the nearest Toyota dealer and had them check it out. They found nothing but one short circuit, around the same time the warning lights would have gone on. I googled the problem, and it seemed like everyone was saying the problem is caused by a bad 12V battery, so I told the dealer to put in a new one. Spent $180 and I was on the road again. However, the first stop I made for gas and sure enough, the light came back and I couldn't get it into a gear without restarting the car several times. I just hope I won't need to spend any more money on this 2 year old car.
Also, the mechanic ... was bad. He said 'B' gear was "Battery Recharge" gear. I said "Really? I read it was supposed to simulate engine breaking." He clearly stated that it will recharge the battery better. Now that I get a chance to sit down with my computer and do a search, everything I find says he was completely wrong.
Yes, many people have reported exactly that message when their 12V battery is low, however in most cases this is only when trying to start (make ready) the prius. The fact that your problem occurred while driving makes it a little different. I'd start by checking that your battery is actually charging (DC DC converter working properly). Measure the battery voltage.
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html - do this after the car has sat for many hours or overnight. Let us know the voltages at each step. (The voltage at the very last step should give us an indication of the DC to DC converter voltage but the numbers in the other step are important too.) You can just flip between parking lights and off instead of going all the way to headlights on.
Where did they say the short was? Their claim of knowing when it happened is curious. Check the battery yourself. New batteries are sometimes stored or charged improperly before installation. With any luck your original was bad and the replacement is simply a lemon.
Don't you mean 4+ year old car? Not really. I had it occur while driving, which is what got me to immediately research the issue and replace the 12V battery. Previously, I would rarely see it while parked, so it was an ignorable nuisance. When it happens while driving and ICE power is lost, it becomes much less ignorable. Anyhow, I agree with the previous posters that you should check the 12V battery again. Here's a link for the procedure to check it: Optima DS46B24R Optima battery direct fit replacement for Prius 2004 +
Was this a salvage title rebuild? Something is causing the DC converter to go offline. If there is a short, that could cause it. Does it have an aftermarket stereo for example? Please check your $180 invoice for diagnostic codes and post them here. Driving the car with the DC converter offline can kill an old 12V battery so it is not entirely money down the drain. I think you will be able to limp home, by trying the following: Go to a Pep Boys and buy a portable jump kit. When you get this message, pull over to the side of the road and turn off the car. You may have to press and hold the button for several seconds. To clear codes, unplug the 12V battery as shown in post #10 http://priuschat.com/forums/gen-ii-...le-vsc-transmission-problems.html#post1363946 Attach jump pack to the battery and turn it on. The 12V is weak and might need help rebooting the car. Re-attached white 12V connector. Foot on brake, push start button twice. By default, the first push is Ig-On after a battery disconnect. Do not leave it in Ig-On because your 12V battery is weak. Disconnect the jump pack -- and charge it every night. Please do also check your inverter coolant reservoir with the car in Ready, to make sure that coolant is flowing. If not flowing, that is the culprit.