My canon eos rebel xti keeps showing error code 99

To isolate the cause of the issue, we suggest that you do the
following:

1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert a fully charged battery, and turn on the camera.

Does the "ERR 99" message appear? If it does, then the camera should
be serviced. If it does not, then please proceed to the next set of
steps:

1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.

Does the "ERR 99" message appear? If so, then the CF card is the most
likely source of the issue. Try using a different card.

If the message does not appear, please complete the following steps:

1. Turn off the camera.
2. Clean the lens contacts by gently rubbing them with a pencil eraser
or soft cloth. Be careful that you do not let any debris fall into the
camera body.
3. Reattach your lens.
4. Turn on the camera.

If the "ERR 99" message only appears when one particular lens is
attached, then that lens should be examined by a service technician.

If you see the "ERR 99" with a different Canon lens attached, then the
camera should be serviced.