The key turns but the door stays shut

There is a particular kind of alarm that comes with a key that turns freely and achieves nothing. A key that will not turn at all feels like a stuck lock. A key that spins feels like a broken one, and people tend to respond by turning it harder, which is the one thing guaranteed to make it worse.

Here is what is usually happening on the other side of the door.

Four failures that feel identical from outside

The cam has sheared. At the back of a euro cylinder is a small flat tab that pokes into the lock case and drives everything else. It is the deliberate weak point. When it shears, the key turns smoothly through 360 degrees and moves nothing at all.

The gearbox has failed. On a multipoint door — most aluminium and uPVC doors, and many newer flat doors — the cylinder drives a gearbox in the door edge, which drives the bolts up and down the leaf. Gearboxes break, especially when a door has been forced against resistance for months. The symptom is a handle that lifts with no weight behind it, or a key that turns without engaging.

Something has snapped inside the case. A follower, a spring, a lever. Less common, but it produces the same helpless feeling.

The bolts are out and stuck. The mechanism worked, threw the bolts, and now cannot retract them because the door has dropped and the bolts are jammed against their keeps. Very common in summer here, when aluminium has expanded.

Only the last of these is sometimes solvable by an owner, and even that has a right and a wrong way.

What to try, gently, once

If the door is closed and will not open:

• Push the door firmly towards the frame while turning the key. If a bolt is binding, taking the load off it can free the mechanism.
• On a hinged door, try lifting the leaf slightly by the handle side while turning.
• If the handle lifts, try turning the key immediately, in one movement, rather than lifting and then hunting for the key.

Each of those is worth one calm attempt. If none works, stop. The next thing that breaks is the cam, or the key, and both make the job longer and more expensive.

What not to do

Do not keep turning. A cylinder key is soft brass. It snaps in a lock without much effort, and a broken key in a jammed mechanism is two problems instead of one.

Do not put oil in it. It will not help a mechanical failure, and in this town oil is what turns airborne salt into grinding paste.

Do not lever the door. The frame costs far more than the lock. A screwdriver mark in an aluminium profile is permanent.

Do not remove the cylinder screw yet. Without the right technique, taking out the fixing bolt can drop the cylinder inside the door and remove the only remaining way of operating it.

What the repair usually is

If it is the cam, the cylinder is replaced and the door is often fine. If it is the gearbox, that part is replaced, and it must match the profile of the door — which is why a photograph of the door edge with the handle lifted is the first thing worth sending. If it is alignment, the cure is adjustment rather than parts, and that is the cheapest outcome of the four.

The pattern worth noticing is that all of these except pure bad luck were predictable. A gearbox that fails in August has almost always been getting stiffer since the previous summer. A door that needs a shove to lock is a door telling you that its bolts and keeps no longer line up.

If your door is doing that now, while it still works, it is a half-hour adjustment. Left until the day it fails while locked, it is an opening, a part and a wait.