Failed update solution(s)
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https://support.usa.canon.com/kb/index?page=content&id=ART136286&cat=EF&actp=LIST
Though this is when a specific lens is attached still might have some relevance?
Description
Recovering from a failed firmware update procedure with the EOS 5D MkIII and EF 40mm f2.8 STM
Solution
For the
EOS 5D MkIII
We have found that the EF 40mm 2.8 STM firmware update failure with the EOS 5D Mark III can occur if the camera has Battery Info Registered and if the Battery Info is checked right before the firmware update is applied.
To restart the firmware update please follow the below procedure.
1. Remove the battery from the camera and remove the lens from the body, do not remove firmware update card.
2. Reinsert the battery and in the camera's menu go to "SET UP3" and select "Battery info."
3. When the battery info is displayed press the "INFO." button found next to the MENU button to display registered batteries and select "Delete info."
NOTE: If "Register" is highlighted that means that no batteries are registered so you can proceed to step 5
4. Delete all registered battery info displayed.
5. Remove battery from camera and reattach 40mm lens
6. Reinsert battery, "Firmware update failed. Try updating again." should appear, select OK to proceed with firmware update. Please ignore Current Version 0.0.0. Update should complete as normal with no issue.
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Hope you can get it sorted out OK