A back door lock that sticks is one of those small things that becomes a big thing on the first cold night, when someone is standing on the step with an armful of groceries. It is worth a few minutes now. Most sticking locks come down to one of three problems: a worn latch, a door that has shifted in its frame, or a key that has worn down. Only one of those usually calls for help.
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Test it first
Close the door and try the key without pushing on the door. If the key turns smoothly and the bolt slides in and out with a clean click, the lock is working, and the trouble is the door or the strike plate. If the key will not turn, turns only with force, or spins without moving the bolt, the lock itself needs attention.
Look at the keys. Try the spare and the original. If one works and the other does not, the problem is the key, and a copy from a hardware counter may fix it. Keep the working key aside before you make any other change.
What you can do yourself
If the door drags at the top corner or rubs against the frame, the hinges are often the cause. Tighten the hinge screws, and if they keep loosening, ask whether longer screws reaching the framing would hold better. If the strike plate is out of line with the bolt, loosen it, move it slightly, and tighten it again. Use a dry graphite lubricant in the keyway and on the bolt, not oil, which collects dust and thickens over time. Do one step at a time and test the door after each.
When to call a locksmith
Call someone if the key will not turn in either direction, if the cylinder spins freely, if the lock has been damaged or forced, or if you need to change who has keys. A locksmith can rekey the lock so old keys stop working, which matters if Mom's set is still floating around the family. Ask for the quote in writing. It should say whether the visit is charged separately from the work, whether they rekey or replace the cylinder, whether new keys are included, and whether the job is finished in one visit. Ask what the warranty covers, the parts, the labor, or both.
Ask whether they are licensed for this work where you live, and ask for a receipt that lists everything done. If the first quote is not clear, get a second one before you agree.
What to do tonight
Test the key without pushing on the door. Note which of the three problems you see. Then make a short list of who has a key: him, you, a neighbor, anyone who has been in the house since the funeral. Put the spare by the phone, and tell him what you found in plain words. Do not take the lock apart in the dark.
When to ask the doctor
A lock is a simple question, but his hands are not. If he is having trouble with grip, steadiness, or strength and cannot work the key or the door, his doctor should hear about it, since it affects far more than the front and back doors. In the meantime, plan to open the door for him rather than leaving him to struggle with it.
Good-week card: The back door opens and closes with one key, and everyone who holds a key knows who they are.