After I changed my battery on my Toyota Corolla 2008, the stereo (Fujitsu Ten stereo) stopped outputting sound (it would still power on). Occasionally it would make the 'sshhhhhhhhh' sound, but this was always random and didn't last very long. I didn't 'SEC' or 'HELP' on the display until I started trying to fix. I eventually fixed it. These are things I tried:
1. Checked all fuses for a blown fuse. No blown fuses.
2. Pulled stereo from the dash. Made sure all connections looked Ok. No obvious issues with connections.
3. Decided to enter SEC code. Wasn't sure what it was, so tried last 3 digits of VIN. Did not work. Tried a number of other codes and eventually got to the dreaded 'HELP' display.
4. Hold button number 1,4,6 and push the power button. This returned to the 'SEC' display. But I still didn't know the code...
5. After removing stereo from dash, I disconnected all connections to stereo, and left it disconnected for a few days. I also disconnected the car battery for a day. Then reconnected battery and stereo...still got the 'HELP' display.
6. Removed the stereo again and disconnected. Removed the face plate of stereo. Put the face plate back on. Reconnected stereo. It worked!
NOTE: When I reconnected in step 5 (above), I forgot to reconnect the round cable (I think it is for the aerial). So I don't know if step 5 was the fix or step 6. I would try both.
