The uPVC multipoint lock is a quietly brilliant piece of engineering, lift the handle and hooks, rollers and bolts engage along the full height of the door. (If yours is a composite door, the slab is heavier and the failure pattern differs a little, see composite door lock problems.) But when it goes wrong, it goes wrong in a very particular way, and the single most expensive thing you can do is force it. This is one of the most common jobs our locksmiths attend, and the same story plays out weekly: a stiff handle forced one time too many, a snapped gearbox, and a repair that's now twice the price it needed to be.
A multipoint mechanism that's getting stiffer is asking for help, not muscle. The gearbox at the centre of the mechanism is the part that converts your handle-lift into all that locking movement, and it is the part that shears when forced. A door with a working-but-misaligned mechanism is a quick adjustment; a door with a snapped gearbox is a parts replacement — and if it snaps while the door is shut and locked, it can become a lockout on top. If the handle is fighting you, treat that as the warning.
This one test tells you more than anything else. Open the door fully, then lift the handle and turn the key with the door standing open.
uPVC doors move. Hinges settle and loosen with years of use, and the material itself expands in hot weather, which is why some doors only misbehave in summer, or only on cold mornings. When the door drops even a few millimetres, the hooks and rollers start catching the edges of their keeps instead of sliding home, and the handle stiffens. The fix is realignment: adjusting the hinges (most modern uPVC hinges adjust in two or three directions), repositioning the keeps, and in more settled cases re-packing the glazing to square the door in its frame, what the trade calls toe-and-heeling. Confident DIYers can often improve things with hinge adjustments alone; the keeps and glazing are better left to someone who does it daily.
This is one of the most common summer call-outs there is, and it has a tidy explanation. uPVC is a thermoplastic: it expands when it warms up and contracts when it cools. A door that has been baking in afternoon sun can grow by a millimetre or two, not much, but a multipoint lock needs its hooks and rollers to line up with their keeps to within less than that, so even a tiny swelling is enough to stop the bolts dropping home.
The tell-tale sign is timing. If the door locks perfectly first thing in the morning but fights you by late afternoon, or only ever plays up in a heatwave, heat is almost certainly the culprit rather than a broken mechanism. The held-open test above confirms it: a door that locks freely when open but not when shut is an alignment problem, and in summer that alignment is being nudged by the heat.
The trade’s own emergency trick works surprisingly well as a stopgap: cool the door down. A cloth soaked in cold water wiped over the slab, or in stubborn cases a few minutes with a garden hose, shrinks the uPVC back toward its normal size and usually lets the door lock so you can leave the house secure. Wait for the evening and the problem often disappears on its own.
But treat that as first aid, not a cure. A door that was fitted and adjusted correctly should cope with a British summer without complaint; one that does not has usually “dropped” a little on its hinges over the years, so it was already sitting close to the edge of its tolerance before the heat finished the job. The lasting fix is a proper realignment, hinge and keep adjustment to give the lock the clearance it needs in every season, which is exactly the work described above. If you find yourself reaching for the cold cloth every summer, the door is telling you it is overdue a small adjustment, not that it needs replacing. The same summer expansion can jam a window shut, too, our guide to freeing a stuck window covers that side.
Gearboxes rarely die without warning. The handle gets gradually stiffer, the lift starts to feel crunchy or gritty, the key needs a jiggle, or the handle stops springing back level. Any of those signs means the internals are wearing — and the good news is that failure doesn't mean a new door, or even a whole new mechanism. The gearbox is a replaceable unit, and in most cases a locksmith can match and swap it the same day, keeping the rest of the strip, the handles and often the cylinder you already have. uPVC windows use the same handle and gearbox arrangement, so if it's a window giving you this trouble, see why a uPVC window won't lock.
The classic oil-based maintenance spray is the wrong tool here: it attracts dust and grime that gum up the mechanism over time. Use a PTFE, silicone or graphite lock lubricant on the moving parts and locking points once or twice a year, and the mechanism will thank you for it.
A named, vetted master locksmith covers your area — no call-out fee.
A straightforward realignment is typically a single visit at the lower end of locksmith pricing; a gearbox replacement usually lands between £90 and £180 plus the part, depending on the mechanism, our price guide sets out fair ranges in detail. One genuine tip while the door is being worked on: if your cylinder is the original builder-grade fit, it is almost certainly snappable, and upgrading it during the same visit costs far less than a separate call-out. Our anti-snap guide explains the two-minute check. If your door won't lock at all tonight, treat it as urgent, an unlockable door is an unsecured home, and it's exactly the kind of job our locksmiths run out to same-day. Find your local LocksmithLocal here.
Most window-locking faults are a quick fix, not a new window. How the espag and gearbox work, what each symptom means, the DIY checks to try, and the part-matching trap.
Past the point of DIY? Find locksmith services near you and book a real, accountable professional.
Broken off, snapped, stuck or won't turn — different problems with different fixes. Why superglue is the internet's worst advice, when pliers are safe, and what professional extraction involves.
Burglars are shoppers, not masterminds — most decide at the kerb. Fifteen prevention measures ranked by value, from the free habits to the hardware that ends the conversation.