A uPVC window that will not lock is more than an annoyance — it is an unsecured opening, and often an insurance condition quietly broken. The reassuring news is that most window-locking faults come down to a handful of causes, several of which you can sort yourself in a few minutes, and the rest of which are a quick, inexpensive job for a locksmith rather than a whole new window. Here is how a uPVC window actually locks, why it stops, and exactly what to do about it.
Never force the handle when the window is not fully closed, and never wrench a stiff handle hard. The locking strip inside is mostly plastic-and-zinc gearing — forcing it is the quickest way to turn a five-minute fix into a gearbox replacement.
Understanding the mechanism makes every fault below obvious. When you lift or turn the handle, it drives a central gearbox, which pushes a long espagnolette (“espag”) strip up and down the edge of the window. That strip carries shootbolts, cams and rollers that shoot out into keeps — metal plates fixed to the frame — locking the window at several points at once. When something in that chain fails, the window either will not lock or will not unlock.
The way it fails tells you where the fault is. Match yours to the list:
Work through these in order before calling anyone — they resolve a good share of cases at no cost.
Clean and lubricate. Dirt and dried-out grease are the commonest cause of stiffness. With the window open, brush out the visible gearing along the edge and apply a little dry PTFE or silicone-based lubricant to the moving parts and the keeps. Avoid thick oils and grease, which only attract more grime.
Tighten the handle. A loose handle is often just two screws backing out — usually under a small cover cap you can rotate or prise aside. Snug them up and the handle may firm up.
Check the alignment. If it locks with the window held open but not when shut, the sash has dropped slightly and the bolts no longer meet their keeps. The keeps usually have a little adjustment in the screws; loosening, nudging and re-tightening them can recover the engagement. On a hinged casement the hinges themselves may need adjusting or replacing if they have worn.
uPVC is a thermoplastic and expands a little in heat, so a window that locks perfectly in the morning can fight you by a baking afternoon — the frame has swollen just enough to push the cams off their keeps. Letting it cool, or wiping the frame with a cool damp cloth, often lets it lock again as a stopgap. If it happens every summer, though, treat it as a sign the window is sitting close to the edge of its alignment and is overdue a proper adjustment.
If the handle spins without engaging anything, the gearbox has gone — the single most common terminal fault on uPVC windows, usually after years of use or from being forced. It is replaceable: the handle comes off, the espag strip unscrews from the frame, and the gearbox at its centre is swapped. The catch is matching the replacement exactly — espag strips vary by length, backset and faceplate width, and some older series (such as certain Mila ranges) are obsolete and need a known modern equivalent. Getting the wrong part is the usual reason a DIY gearbox job stalls.
If a key has snapped off in a locking handle, do not poke at it — the same careful extraction that applies to door locks applies here, and is covered in our guide to a key broken off or snapped in the lock. If the handle turns but feels disconnected, the square spindle linking handle to gearbox may have worn or sheared, which means a new handle.
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Call in help if the handle spins uselessly, the window is jammed shut, you cannot identify the right replacement part, or you simply would rather it was done once and done right. A locksmith who works on uPVC daily will diagnose it in minutes, carry the common gearboxes, handles and keeps, and — crucially — will get the security back to the standard your insurer expects. It is almost always a repair, not a replacement window: see what that typically costs in our UK locksmith price guide, and once it is sorted, our window security guide covers keeping every opening properly locked.
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