Electrical problem
2000 Buick Regal 6 cyl Automatic 169000 miles
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The right front door no longer locks. But I can still lock all the doors with the lock button.

Probably a switch or actuator problem.
This is the test for the switch:






When you say the lock button operates the lock, do you mean the key FOB?

yes, the door button and the fob will lock all the doors except the right front. The key also does not lock the door.

Okay, now I'm with you. Can you hear the actuator working on that door?

Either way you'll have to pull off the door panel and get a look inside. There may be linkage disconnected or a seized actuator is what I would expect you to find.

Yes, I can hear something.

Then you need to pull the panel off and get a look at the linkages inside. Keep the window in the up position to allow access. This sounds like a mechanical isue where something has come loose or is binding.