Canon EOS 40D Instruction Manual page 175

Shooting
No images can be shot or recorded.
The CF card is not properly inserted.
If the CF card is full, replace the card or delete unnecessary images to
make room.
If you try to focus in the One-Shot AF mode while the focus
confirmation light <o> in the viewfinder blinks, a picture cannot be
taken. Press the shutter button halfway again to focus, or focus
manually. (p.32, 80)
The LCD monitor does not display a clear image.
If dust is adhering to the LCD monitor, wipe with a lens cloth or soft
cloth.
In low or high temperatures, the LCD monitor display may seem slow
or it might look black. It will return to normal at room temperature.
The image is out of focus.
Set the lens focus mode switch to <AF>. (p.30)
To prevent camera shake, hold the camera still and press the shutter
button gently. (p.32, 35)
The CF card cannot be used.
If a CF card-related error message is displayed, see page 43 or 177.
The camera makes a noise when it is shaken.
The built-in flash's pop-up mechanism shakes slightly. This is normal.
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p.28, 124
Troubleshooting Guide
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175