Why won't your uPVC door lock properly, and what to check first

Step one: the lift the handle test

Close the door fully so it's sitting in the frame, then lift the handle firmly before turning the key. Multi-point locks need the handle lifted to drive the hooks or rollers into the keeps; if the handle lifts easily and the key then turns smoothly, the lock mechanism itself is probably fine and the fault is elsewhere. If the handle feels stiff, gritty or won't lift the last few millimetres, that points toward the gearbox or alignment rather than the cylinder.

Step two: check hinge and keep alignment

With the door open, look along the gap between the door and the frame at the hinge side and the lock side. An uneven gap, wider at the top or bottom, usually means the door has shifted slightly on its hinges. Adjustable hinges can be nudged back into alignment with an Allen key, moving the door up, down or sideways in small increments until the locking points line up with their keeps again.

• Open the door and sight down the gap on both the hinge and lock side
• Identify whether the gap is uneven top to bottom
• Use the hinge adjustment screws to bring the door back central
• Retest the lift and lock action after each small adjustment

A dropped door versus seasonal expansion

A door that has genuinely dropped, usually from hinge wear or a slightly sagging frame, will show a consistent gap change that doesn't come and go. Seasonal expansion is different: uPVC and composite doors both expand slightly in hot weather and contract in the cold, so a door that locks fine in winter but binds in a heatwave, or vice versa, is often just doing what the material does rather than showing a fault.

If it's seasonal, a small hinge tweak for the season is a reasonable fix; if it's a genuine drop, the hinges or their fixings usually need proper attention rather than a repeat adjustment every few months.

Worn gearbox: the most common failure

Inside the door edge, the gearbox is the mechanism that translates lifting the handle into the hooks, rollers or bolts firing out. Gearboxes wear out over years of daily use, and a failing one often shows up as a handle that feels loose, drops back down on its own, or lifts without any resistance at all, meaning the locking points aren't actually being driven out.

This is a mechanical part with a working life, not a sign the door itself has failed, and it's a straightforward like for like swap once the correct gearbox backset and length are identified.

Snapped spindle and cylinder problems

If the handle spins freely with no resistance at all and does nothing, the spindle connecting the two handles through the gearbox has likely sheared, a cheap part to replace but one that needs the correct spindle length for your door thickness. Separately, if the key turns but the door still won't lock, or turns very stiffly, the cylinder itself may be worn or damaged, which is also the point at which it's worth checking whether it's a basic cylinder or a TS007 3 star rated one, since replacing a failed cylinder is a sensible moment to upgrade it.

Keeps and strike plates worth a look too

The keeps set into the frame, the metal plates the hooks or rollers drive into, can themselves work loose over time as screws in uPVC or timber frames back out slightly under repeated use. A keep that has shifted even a millimetre or two can be enough to stop a hook engaging fully, and it's a quick thing to check with a screwdriver before assuming the gearbox itself has failed. Tightening loose keep fixings costs nothing and rules out one more possibility before calling anyone out.

A quick checklist before you call anyone

Running through a short list first often narrows down the problem, or resolves it outright, before it becomes a call out.

• Lift the handle firmly with the door closed and note whether it lifts smoothly
• Sight down the door gap on the hinge and lock side for unevenness
• Check whether the fault only happens in hot or cold weather
• Tighten any visibly loose keep or hinge screws with the door open
• Note whether the key turns freely but the door still doesn't lock

When a gearbox swap beats a new door

In the large majority of non-locking door calls, the frame and panel are sound and only the gearbox, hinges or cylinder need attention, which is a fraction of the cost of a new door. A full replacement only makes sense where the frame itself has failed structurally, the door has been forced or damaged, or the glazing has misted alongside the lock fault, at which point it's worth weighing repair cost against our doors from £799 fully fitted.