Door locking problems - explained

[randy500]

Every few weeks someone has a question about their door being hard to lock with the key or unable to lock with the key. They usually don’t mention the knob is hard to lock too but that is inherent with the door key locking problem since the key and the knob interact on the same offending part.
Usually the problem arises after installing new door weather stripping.
Here is a picture of a latch that I cut away to depict the problem.
I have labeled the parts with a sharpie.
1, is the door latch pawl
2, is the door lock pawl
3, is part of the lock pawl which when locked, prevents the outside and inside handles from disengaging the latch pawl.

In the center of the picture, rotating on the round pin is the “toothed” latch/lock cylinder. This part is one piece and from this view you can see the “teeth” which are beveled on one side and square cut on the other. This part is rotated by the gear like teeth on the outside of the door which engage the striker plate on the door jamb.

The first picture shows the latch unlocked, the second picture shows the latch locked.