Pictures taken with my 1D Mark IV show a blue horizontal line at the bottom from ISO 400 onwards.

Question

Pictures taken with my 1D Mark IV show a blue horizontal line at the bottom from ISO 400 onwards. So far nobody has been able tot tell me the cause of this problem. The body is still quite new (15000 pics taken).

Answer 1

I agree that it looks like a column of photosites is not working.  But... I'm curious as to why this would only happen at ISO 400 or above. 
The sensor is covered with photosites -- they are monochrome.   On top of the photosites is the "bayer mask".  The mask is a checkboard pattern of red, green, and blue "lenses" which makes the photosite beneath only sensitive to that particular color.  They are group in clusters of 4 photosites in a 2x2 matrix.  Two of the four squares (in opposing corners) are green.  One corner is red.  One corner is blue.  If you were to lose a column which happens to be along the red/green column (as opposed to the green/blue) then you'd lose red sensitive and half the green sensitivity in that column.  The result would be that this column is flanked by two "working" columns which are full of green/blue photosites... hence the teal blue look.
I do not know if this will work for you, but oddly... you can actually sometimes "clear" stuck pixels (but usually this is an indvidiual stuck pixel... not a whole column of them) by putting the camera into manual sensor cleaning mode (and you may have to do this 2 or three times).  While the normal point of the self-cleaning mode is to flip up the reflex mirror and open the shutter so that you can inspect and clean the sensor (really the front-most filter because you can't actually touch the real sensor), it also neutralizes the charge on the sensor (or so I've read).  This charging and discharging of the sensor a few times can often get "stuck" pixels to start working again.  BUT AGAIN... I've only ever seen this used on individual stuck pixels... never a whole row.
Tim Campbell
5D II, 5D III, 60Da