I had my Kenwood 440 go into protect mode a few weeks back while I was doing some work and listening to the stereo. Kinda freaked out, tried a few different things like, disconnecting the battery, popped the deck head off and pushed the reset button, each time it would play for a second or two and go back into protect mode. Then I relized the battery only had about 9 volts left in it. Charged battery for a few minutes, pushed the reset button again everything was back working. Mandley may be right if you have a battery with a dead cell it could cause an issue with the HU.