For Euro-profile and oval-profile mortice cases, the keyed part is a cylinder held by a single fixing screw through the faceplate. Take the screw out, turn the cam into line with the cylinder body, and the cylinder comes free without dismantling the case. A rim lock bolts to the inside face of the door as a visible box, and its cylinder is held from the inside, not from the edge of the door. A mortice or Euro-profile case sits inside the door edge, so the faceplate screw is the thing to find.

Whether a cylinder swap is on the table depends on that design. A locksmith can tell which you have on sight, and a clear photograph of the faceplate and the front of the lock lets you narrow it down before the visit.

A few checks cost nothing and help separate a keyed-part fault from a case or door fault. Work through them in this order:

The cylinder holds the plug, the pins, and a small cam at the back. When the right key lines up the pins, the plug turns and the cam moves the mechanism in the case, and the case draws the latch or throws the bolt. Every key that goes in wears the plug and pins, while the case behind it sits idle until the cam turns it. That is why a worn or damaged keyed part can be replaced while a sound case is kept.

In a cylinder swap, the case, the bolt, the strike plate, the handles and the holes cut in the door all stay. What goes is the part that recognises the key. The new cylinder must fit the case it goes into, so the locksmith checks the existing hardware before ordering. A cylinder that fits the opening can still fail to drive the case.

The work is quick and leaves the door as it was. There is no repainting around a new plate and no re-cutting of the timber, because nothing the door was prepared for has moved.

A new cylinder cannot reach a fault that lives behind it. The test is to take the cylinder out and work the case on its own: if the bolt still drags, sticks, or slops about with the cylinder removed and the door open, the wear is in the case, and the whole lock is the job.

Three other findings point the same way. A door that has been prised or struck can hide a bent bolt or a cracked case behind a cylinder that still turns, so the lock and the timber around it are checked together. Where the keying is built into the case, there is no cylinder to swap. And where the shop wants the door to do something new, such as deadlocking from outside where it only latched before, the case has to change to give it that function. Bent or cracked handles and escutcheons go with the lock they were made for.

A whole-lock replacement also gives the chance to change the type of lock if the old one no longer suits the door. A shop that has outgrown a simple latch, or an office that wants a lock with a different bolt, is choosing hardware at that point as well as repairing.

A cylinder swap and a whole-lock replacement change the keys in the same way because, on a cylinder lock, the keying lives in the cylinder either way. The new cylinder can come with its own keys, so every key to the old one stops working, or it can be pinned to the key the business already uses, which keeps the keys and only changes the worn part. A missing or unreturned key decides which: if one is still out there, the door needs new keys.

For an office or shop with more than one door, the question is how the new cylinder is keyed to the others. A new cylinder that arrives with its own key set will not match the front, back, and storeroom doors, which still carry the old cylinders. The choices are:

- Keyed alike: The new cylinder is made to take the same key as other doors, so one key still opens the set.

- Master keyed: Each door keeps its own key, and a master key opens all of them, which suits a site where staff should reach only their own area.

- Its own key: The door becomes a separate key from the rest, which suits a storeroom or a till office.

Ask for that decision to be made at the time of ordering, because adding it afterward means another visit. For a business that has lost a key or had staff leave, a changed cylinder removes the departed person's key from that door in one job. Rekeying the cylinder already in the door is a third route, which keeps the hardware and changes the pins; a new cylinder is the choice where the keyway or plug itself is the worn part.

Call when the screw cannot be found, when the key has snapped in the lock, when the bolt will not move with the door open, or when the door has been forced. Call also when the lock cannot be made to secure the door, since a shop door held overnight by its latch alone has no deadlock behind it. The locksmith removes the cylinder, checks the case and bolt behind it, and says on the spot whether the swap will do.

Have the photographs, the key count and a note of how the lock behaved open and closed ready. Those three things let the right cylinder or lock be brought to the first visit.

Find which part has failed before buying anything, and a small job stays small.