Error 5 has two conditions. One is no flash pop up and the other is pop up but no flash.
Both are likely caused by dirt. You can use denatured alcohol on a Q-tip to try and clean the linkage and the very small sensor pin.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
