The key goes into the lock the way it always has. Then it will not turn, or it moves a small amount and stops.

Most people push harder at this point, and that is worth holding off on. A key under pressure is the one most likely to break off inside the cylinder.

Most cases come down to one of three causes. Either the door has moved, or the key has worn down, or the cylinder inside the lock has worn out. There is a ten second test that tells you which one you are dealing with, and it is worth doing before anything else.

Start by Opening the Door and Trying the Key Again

You need to know whether the lock is the problem at all before you look at anything else. A door that has shifted on its frame can feel exactly like a worn cylinder at the key. The quickest way to tell is to try the lock with the door open.

Swing the door wide open. Put the key in and turn it while the door hangs free. Then see which of these two things happens:

Another clue is a key that only turns when you pull the door toward you, push against it or lift the door slightly. That usually points to door or strike alignment rather than the key itself.

There is a third possibility worth ruling out while you are there. If the key turns all the way around with no resistance and the bolt does not move at all, the problem is probably not the key. The connection between the cylinder and the bolt has failed, usually the flat bar that carries the turn through the door. A new key will not help with that one and neither will lubricant. It needs the lock taken apart.

Most of the Time It Is the Key, Not the Lock

This is the cheapest possibility, so it is worth checking before you do anything to the lock.

Key cuts wear down with use. Every time the key goes in, the brass rides across the pins inside the lock, and over the years the sharp edges of those cuts round off until they sit slightly shallow. The lock is unchanged, but the key no longer matches it.

Duplication speeds this up, because a copy is traced from whatever key you hand over the counter. Copy a copy and you carry the first key’s wear plus the tracing error, and after a few generations the new copy may no longer match the lock closely enough.

Find a different copy and try that instead, ideally the original that came with the lock or a spare that has spent its life in a drawer. On service calls it is common to find the drawer spare turning perfectly while the everyday key barely moves, and that usually settles the question before anything gets taken apart.

When the Wear Has Moved Into the Cylinder

If every key you own struggles, the wear is inside the lock. Pins shorten, springs weaken, and the tolerances that made the cylinder work start to drift.

How far along that wear is matters, because a cylinder that has only just begun to drift can often be serviced, while one that has been ignored for a year is usually past that. A worn cylinder usually gives you some warning before it stops working:

A worn cylinder usually gets harder to use over time. It may need some jiggling at first, then eventually stop turning altogether. This usually shows up first on the door that gets used most, which is often the front door.

What to Put in a Lock, and What Ruins One

If the lock is dry rather than worn, the right product frees it in a few seconds. The wrong one does damage that only shows up months later, and this is the most common way a stiff lock gets made permanently worse. It is worth knowing which is which before you spray anything into the keyway:

There is a limit to what any lubricant can do. If lubricant helps only temporarily and the stiffness comes back, the lock may have wear or another mechanical problem that lubricant cannot fix.

The Quick Things Worth Ruling Out

If the problem appeared suddenly rather than getting worse over time, check the key, keyway and door alignment before assuming the cylinder has worn out. Wear is the most common answer but it is not the only one, and the alternatives take about a minute to eliminate. It is worth going through them before you decide the lock needs work, because a couple of them cost nothing at all to fix:

If none of that applies and you have no working key at all, a locksmith can still make one from the lock itself.

When to Stop Turning the Key

Brass is soft, so a key under real pressure bends before it moves the pins, and once it has bent it can break with part of it left inside the cylinder. At that point you are looking at extraction before anything else can happen.

Put the key down if it flexes visibly under pressure, if it turns partway and hits something solid, or if it only works from one particular position that you have got used to finding.

From there it is a service call, and there are two things worth asking when you make it. Whether the cylinder can be serviced or has to be replaced, and whether the other doors are running on the same key. If replacement turns out to be the answer, rekeying the new hardware to your existing key is usually possible and saves carrying a second key around.

Get the Lock Looked At While the Key Still Turns