We have been notified about a known issue within Canon on the Canon EOS-1D X and Canon EOS-1D C camera bodies and sometimes not autofocusing in cold weather situations. I have read about the issues on our own forum as well as on others.
Below is an explanation of the issue we've received from an anonymous source, though no official service advisory has been issued by Canon.
Camera “does not autofocus”, “does not search in AF”, or “does not focus in AF search” depending on the reporter or the camera settings; specifically, the focus cannot be achieved in low temperatures (under 0 degree Celsius).
* Although there has been no reported occurrence on the EOS-1D C, the mechanical structure of the Mirror Box Ass'y is the same as the EOS-1DX, so the EOS-1D C is included in the affected product.
This phenomenon is due to the Locking Claw of the Sub Mirror (mirror for AF) going over the Locking Pin. The Sub Mirror's angle becomes deviated and the light rays for AF does not fall on the AF sensors, causing the “does not autofocus” phenomenon.
Service of Affected Product:
To control the torque, the process to check the Locking Claw's going over torque of Locking Pin has been newly incorporated. (From production on January 24)
Handling of units in question: If the user complains about this specific phenomenon (does not autofocus in low temperature), replace the Mirror Box Ass'y with the ones to be shipped in the future (CY3-1661-010 or CY3-1687-010).
Handling of general repairs: If the user does not mention this phenomenon, please handle as normal repair.
The line directly above is of great concern to me. If you bought one of the earlier EOS-1D X cameras and live in a warm climate, you may never know the issues existed until you went to shoot in a cold climate.
The notice is dated January 2013, so I assume all cameras manufactured after that date would be unaffected. There is no serial number range that I can see, nor do we know how widespread the issue is. However, if you have experienced this phenomenon, you now know what it is.