Err 01 on Rebel XSi

Most of the Time when this happens it's the curcuit board inside the lens that bad from use, Needs to be replace by a repair Tech! Try a different Lens to confirm that! TCCameraRepair.com

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Err 99 could mean several different things, such as... Memory Card, Lens or Lens contacts....

Here's what you can try:
Remove the memory card an enable "shooting with out memory card" and see if you still get the error. If it works your memory card is bad. Try another lens if you can. Clean the lens contacts with a pencil eraser (be careful not to get eraser dust in the camera body or in the lens). Worst case scenario is, Err 99 could be related to the shutter failing which would mean a trip to Canon.
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From working with third party lenses in the past what I've found is that Canon doesn't give up the "chip" technology so the older lens can not talk to the newer cameras. I suspect that Canon does this to promote their lenses over the competition. You might be able to send the lens to Sigma and have them re-chip it and have a great lens again.

It could well be the lens, Take it off and clean the contacts on the lens and body,

The Flashing is a normal thing for the Camera to do, Nothing to worry about.

u have lens problem is u see any error in display error 99

To NOT get that error you need to use in AV mode. It's the only way that old Sigmas work with the new Canons. It's an aperture bug that old Sigma lens have and some the only way to fix it is to rechip it. The problem is that Sigma no longer rechips the lens.

Err 99 usually means your lens contacts are dirty either on the lens itself or on the camera.

To isolate the cause of the issue, 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 only a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, the camera may be faulty and requires a service. If it does not, then: -
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, then the CF card is faulty. Try a different card. If the message does not appear: -
1. Turn off the camera.
2. Clean the gold contacts on the lens by GENTLY rubbing them with a pencil eraser or soft cloth. Keeping the camera face pointing down, also clean the gold contacts on the camera, being careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.

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 is faulty and should be serviced.

In the mode option u select sunrise or sunset noe be carefull while taking the photo the direct sunrays should never hit on your lense get a shade above your lense now try to take the photo without flash

its probably a lens issue, possible dirty contacts