Hi,There are a few things that can cause this error. If the flash does not "pop up" when the camera is used in low light (depending on the mode of operation) Err 05 may be displayed. First, there is a known problem with a sensor switch located in the hotshoe (external flash mount). If this gets stuck, dirty or blocked, the camera thinks that an external flash is mounted and stops the "pop up" flash from operating correctly. Use a toothpick to exercise the switch pointed out in the picture, then a brush to clean under the slides (see below) Try this procedure first, take a few shots and see the result.
Next, The spring latch on the pop-up flash can stick. It is part of the hinge in the rear. Once you have raised the flash, you need to clean with a little alcohol and lubricate the latch and spring with WD40 or similar since dirt and grit can inhibit the correct operation.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well.
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