What troubleshooting steps can I take to resolve the Canon 40D Error 99 issue?

To resolve the Canon 40D Error 99 issue, you can try the following troubleshooting steps:

1. Turn off the camera and remove the battery for a few minutes, then reinsert it.
2. Clean the lens contacts and camera body contacts with a soft, dry cloth.
3. Try using a different lens to see if the error persists.
4. Update the camera firmware to the latest version.
5. Reset the camera settings to default.
6. If the issue persists, consider contacting Canon customer support for further assistance.