by 
The Curious Bag
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Digital cameras, including the Canon EOS 30D, can simplify the picture-taking process in a number of ways. But as they are more sophisticated than older cameras, they can also have glitches standard cameras do not. When this happens, an error message will be shown. Error 99 is one you might see.
Lens Contacts
One possible cause for seeing Error 99 is the lens contacts on the rear part of the lens needing to be cleaned. Clean them with a pencil eraser if this is the case.
Battery
A failing battery might be another reason for the Error 99 code. Turn off the camera, and remove and replace the battery to see if the error code goes away.
Accessories
A third-party accessory might also cause Error 99 to appear. Disconnect it and replace it with an official Canon accessory to solve the problem.
Lenses
Unauthorized lenses might lead to an Error 99 code. Only attach a Canon EF or EF-S lens to avoid getting the error message.
Service
If the error code persists after taking these troubleshooting tips, the camera might be in need of service. Contact Canon or your camera retailer to begin this process.
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