Canon Rebel XS (1000D) - Err 99 and an Exploration of the Memory Card Door Switch
A friend recently bought a used Canon 1000D / Rebel XS / Kiss F so her kids could try DSLR photography. The XS/1000D is an ancient (2008!) low-end DSLR with very simple features and no video capability, but its a decent starter body and dirt cheap now (<$100).
The camera has developed the common Err 99 problem. You can shoot photos normally when its working. Generally, once you power it up, you get the the error 99 message (below) after the first shot. They would just remove the memory card and battery, reload and shoot normally until it happened again.
I Googled the error, played around with the camera and replicated the error. Typically, after you switch it on and fire the first shot, you get the Err 99 error. By trial and error I figured out that just sliding open the memory card door and snapping it shut would fix the problem for awhile.
I tried blowing some air through the memory card door micro switch at the bottom of the card door but there was no change. The standard 18-55mm IS II kit lens appears to work fine, and the 99 error showed up with a variety of basic and L lenses, so I don't think its the lens or mount. Since the camera was used, well out of warranty, and not worth the $300 base charge for Canon Canada to look at it, I figured I might as well take it apart and see if there was a micro switch contact that was dirty inside.Unable to find a clear photo of the SD memory card door micro switch on a Canon Rebel XS, I found this webpage that showed getting to the inside of the back panel was just a matter of removing 8 screws and one ribbon cable. 
I removed the 4 screws off the bottom plate and 2 each from the left and right sides and gently pried the back off - it came of easily once I removed the USB/video panel, which just pulls out.
The metal piece contacts a metal pad the circuit board when the memory card door is open, causing the camera to shut off, presumably to prevent a short circuit while inserting or removing the memory card.
When the memory card door is closed and slides shut, a prong on the card door pushes the metal sprung piece up, lifting it off the contact pad on the circuit, indicating to the camera that the door is closed and it is safe to power up. (Below, I've lifted the metal piece up to show the contact pad.)
Basically, I cleaned the contact pad and spring contact with alcohol, put it together, but the Err 99 still appeared. Then I put some tape between the contact pad and the metal piece from the contact pad to the screw to keep the circuit open, but the Err 99 still happened. The metal sprung piece is attached with a single screw, but it is under the LCD screen shield, so it cannot be easily removed. I was going to do this, but since the tape idea didn't work, I suspect that its likely the issue is somewhere further up the circuit board in terms of sensing the voltage at the memory card door switch.
I can't be bothered to disassemble it further. The camera works fine otherwise, and you just have to slide the memory card door open and slide it back to click and it works again: you don't even have to turn the camera off or take out the battery, so its a quick operation. Good enough for now, but at least I know what the mechanism looks like inside. If you see this post and have a solution, let me know. (Aug 2017)
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