The sensor for the flash is actually the clear frame around the flash tube. There is a black 'wire' attached to it (actually a light pipe) that attaches to the frame and then goes back to the 'sensor' at the rear of the flash base (the part that flips up). Pop up the flash and remove the screws underneath to remove the top cover of the flash and make sure the 'wire' is in place. Caution: if the flash is on or has been used before there is over 300 volts across the flash tube. Do not touch either end of the flash tube or you will get a nasty shock.