2002 Prius - Error P3120

Help!

I am getting the nasty warnings on my dash. PS/Battery/CAR! (the car is the highlighted of the three icons). After starting up/shutting down a few times the error went away. It came back the next day (both times it popped up was on the freeway if that matters). I acquired one of those cheapish OBDII scanners they sell at Kragen/Autozone/etc... that shows you the code and allows you to clear it. It told me I have a P3120. I cleared it and its gone at the moment, I am assuming its going to come back later. The car is appearing to run fine with or without the error currently.

Doing some searching on the magical internet thing it looks like I need a fancier scanner to tell me the subcode that would help to narrow down the problem. Since I don't have one of those and I don't want to pay my friendly toyota dealer tons of money to spend 3 minutes with their diagnostic tool......... Is there anything I can do prior to taking it in? Clean the windows... Jump on the bumper...

I checked under the hood yesterday and all the fluids looked fluidy. To the normal lows/highs of where they should be. Beyond that I haven't done anything. Any suggestions?

Thanks for any help (and hopefully saving me money and my car from imploding!)