Took my gadget at Canon Service Center a while ago which I sent last Jan 5, 2009 for dust cleaning. I make test shots at the Service Center and worked fine without any problem.

When I got to my place and open my camera to take some shots, I got this "Error 99" on my display. This sucks a little bit. I just need to go back again and have my 400D fix.

The error doesn't came from the lens, CF card and/or battery. I did try to follow the instruction from the error itself but still no luck at all. Until I came to notice the small internal battery. I reset the system by taking off the internal battery. Put on the camera main battery and it did power off without showing the error.

After a few tweaks, I've finally found the source of the "Error 99", the culprit is the Auto-Sensor Cleaning System during system power down. Finally I disabled the auto cleaning process during power down and the error didn't show up again.

Root Cause: The Auto-Sensor cleaning makes the Error 99.
Temp Fix: Disable the auto-sensor cleaning system.