Do you have a Doromatic Exit and key cylinder that is not working?  Chances are the Doromatic Pinion Gear (Cam) PB5 is missing a tooth or two.  The gear is made of a die cast material, that tends to fail quite often, especially if you are trying to dog your door open with the key cylinder.  The keyed rim cylinder, which has a tab off the back of it, fits into the Pinion Gear (Cam).  When the key is turned the Pinion Gear (Cam) rotates and moves the retractor up and down.  The retractor has teeth on it that the Pinion Gear (Cam) fit into.   The Retractor is hardened steel and very rarely fail.  Another problem might be the Doromatic Pinion Cylinder Bearing.  This is a plastic bushing that takes up the slack in the back plate, I would recommend replacing this also.

The above image shows the dimensions of the Pinion Gear PB5. Teeth tend to break off and cause the key cylinder not to work.

the above image is the Doromatic Pinion Bushing. If this is missing, the Doromatic Pinion Gear will break more easily