Symptom: Outside door knob turned but not engaging. Couldn't open door.
Fix: (after the door is opened,)
1. Take out inside knob and plate (3 screws total) and the cam - the square profiled axle that turns the lock, watch out to not lose the spring on cam.)
2. Take out the striker plate's 2 screws.
3. Take the remaining lock assembly off the door from the outside.
4. Start taking the assembly apart slowly. From the attached picture, see lock01.jpg, the beginning of this step.
Notice the little metal stud on the housing near a screw. That's the only stud on the plate, and the bolt when assembled should be pointing the opposite way.
5. After the screws are removed, the bolt and its plate comes off, watch out not to lose 2 springs in this part. Then the copper cylinder plus its plate below. See lock02.jpg.
6. Now the exposed plate can be taken off. This plate controls the locking from the inside knob. It should go back on in the same position.
7. Now we're at lock03.jpg, exposing the problem.
Now on top is a washer. It should be held on by the metal in the center being flared out. Underneath that washer is the movement plate for the outside knob. When the washer pops loose, the movement plate will also be loose. Its result is the outside knob will not work.
The fix is to push the washer back in, and flare the metal out again to hold the washer in securely. See lock04.jpg
I used whatever happened to be handy in the garage. Put the knob on a piece of soft wood that has some give so the knob is not damaged in the process.
8. Doublecheck when done, to make sure that this time the washer is not going anywhere. Clean up and lube moving parts with grease. See lock05.jpg.
9. Reassemble in reverse order.
The only caution I can think of would be to do it slowly/carefully. In the last picture one can see a rectangular shaped part. That part helps lock the door by turning the knob when inside. It slides left to right and is held in position by a spring and a ball bearing. I'd hate to see these parts lost, or any of the other springs.
I hope this helps anyone who faces this little problem. This procedure should pose no threat to the integrity of your airstream security. You have opened the door to get to the lock, after all.
Please feel free to add info or correct any mistakes.