I have an HCH 03. We had many problems with IMA/transmission and Honda finally replaced the Transmission just after the warranty expired since we had brought it in to the local dealer and others 10 times (maybe more) with the same problem. Ironically we never had any indicator lights go off, but the car shuddered horribly and studdered on down hills yet both were random. Since the trans replacement the IMA and Check Engine lights have come and gone off repeatably. A Honda service tech told me to do nothing (don't waste your money), but suggested I disconnect the regular battery and connect the terms together for half an hour to clear the indicator lights. That usually works for a few days and sometimes weeks. Since that is a hassle, I just ignore the lights or pretend it is Christmas all year and just enjoy the pretty lights.

On a related note, I just got a recall notice about the Check Engine light and a software fix. I think I will give this a try. Wish me luck. BTW Honda, my Prius has never had any issues.