Is this a new to you machine? Has this machine been working and now quit? Have you made any recent changes prior to this problem? Do you have the proper size coin guide in the machine? If not, under the comparitor is a optics assembly. Most likely it is dirty and needs cleaned. It has a sender on one half and a receiver on the other half. They are aligned with holes in the plastic coin guide. Take the assembly out and clean the optics with just a water dampened cloth. Also check the holes in the plastic coin guide .