If you have already replaced the euro cylinder and your UPVC door still will not lock, the fault almost certainly sits in the gearbox or the keep alignment rather than anything to do with the new cylinder itself. A cylinder controls whether the key turns, but it is the gearbox that physically drives the hooks and rollers into the keeps along the frame, and if that part has failed or the door has dropped slightly, a brand new cylinder will not change the outcome at all.

This is one of the most common and most frustrating repair experiences homeowners describe. The cylinder is the cheapest, most visible part of a UPVC lock, and it is also the easiest to buy and fit without calling anyone out, so it is understandably the first thing people replace when a door stops locking properly. The trouble is that a stiff or unresponsive lock is often telling you about a completely different part of the mechanism, and swapping the cylinder does nothing to fix that.

Why the Cylinder Gets Blamed First

A euro cylinder is the part you physically insert the key into, so it feels like the obvious source of a locking fault. It is also sold cheaply online and in hardware shops, comes with straightforward fitting instructions, and can usually be swapped in under fifteen minutes without any specialist tools.

None of that makes it the right diagnosis. The cylinder’s only job is to let a correctly cut key rotate a small cam inside it. Everything that actually happens next, driving hooks, rollers or a deadbolt out into the frame, is handled entirely by a separate component called the gearbox, sometimes referred to as the lockcase. If the gearbox has failed, or if the door itself has dropped and no longer lines up with the keeps in the frame, the cylinder can be in perfect working order and the door will still refuse to lock.

The Three Parts of a UPVC Lock, and Which One Actually Failed

A UPVC multipoint lock is built from three separate systems working together, and understanding the difference is the fastest way to stop paying for the wrong part twice.

The euro cylinder is the section you turn the key in. On its own, it only ever controls rotation of a small internal cam, nothing more.

The gearbox sits inside the door edge and does the mechanical work. When the handle is lifted or the key is turned, the gearbox drives a set of hooks, rollers or a deadbolt outward simultaneously along the length of the door. A worn, cracked or seized gearbox is one of the most common reasons a door will not lock even with a brand new cylinder fitted.

The keeps, sometimes called strike plates, are fixed to the frame itself and are what the hooks and rollers actually slot into when the lock engages. A door that has dropped slightly on its hinges, even by a couple of millimetres, can throw every keep out of alignment at once, which produces exactly the same symptom as a broken gearbox from the outside.

The Symptoms That Actually Point to Each Fault

A handle that lifts freely but the hooks never move at all, with no resistance and no clicking sound, usually points to a gearbox that has failed internally rather than anything related to the cylinder.

A handle that needs to be forced or lifted at an unusual angle before it will engage, but does eventually lock, is a stronger sign of a keep alignment issue caused by the door dropping rather than a gearbox failure.

A key that turns smoothly in the cylinder with the door open, but the door will not lock once it is closed, tells you the cylinder itself is fine and the fault only appears once the hooks try to meet keeps that are no longer sitting where they should be.

A handle that feels loose or floppy, rather than stiff, and does not click into place properly, points toward a different fault again, usually a worn spring cassette inside the handle mechanism rather than the gearbox or the cylinder.

Why Replacing the Cylinder a Second Time Will Not Help Either

It is a natural next step to assume the first cylinder was faulty and try another one, but if the original diagnosis was wrong, a second cylinder produces the exact same result as the first. This is the point where homeowners often end up frustrated and out of pocket for two cylinders that were never going to fix a gearbox or keep a problem in the first place.

The quickest way to avoid this is a simple open door test. With the door wide open, lift the handle and watch the hooks and rollers along the edge. If they move out fully and smoothly on their own, the mechanism inside the door is working correctly, and the fault lies entirely with the keeps on the frame side not lining up when the door closes. If the hooks barely move, move unevenly, or do not move at all, the gearbox itself is the part that has actually failed.

What a Southampton Glazier Actually Checks First

A proper callout for this exact complaint starts with the same open door test described above, since it takes under a minute and immediately narrows the fault down to either the door side or the frame side. From there, a worn or seized gearbox is replaced as a single unit, which is a like for like swap rather than anything involving the cylinder that has already been fitted.

Where the hooks and rollers move correctly but the keeps do not line up, the fix is a keep adjustment, repositioning the strike plates on the frame by a small amount, or in more pronounced cases, adjusting the hinges to lift the door back into proper alignment. Properties across Portswood, Bitterne and Swaythling with older UPVC installations see this fairly often, simply because doors settle slightly over years of daily use, and it has nothing to do with the cylinder that gets blamed first.

A locksmith or glazier who checks against the technical standards published by dhf, the UK trade body for locks and building hardware, will always run this open door check before recommending any replacement part, precisely because it prevents exactly the situation this post is about, paying for a part that was never going to solve the problem.

What This Actually Costs to Put Right

A gearbox replacement is a genuinely different job to a cylinder swap, both in parts cost and labour, but it is still a targeted repair rather than anything close to a full door replacement. Most UPVC doors use a standardised gearbox size that a glazier carries as stock, which keeps the callout straightforward once the fault has been correctly identified.

A keep adjustment is typically the cheaper of the two fixes, since it involves repositioning existing hardware rather than replacing a mechanical part, though a door that has dropped significantly may need a hinge adjustment alongside it to stop the problem recurring within a few months.

Either way, the cost of correctly diagnosing the fault first is almost always lower than the combined cost of a cylinder that was never needed plus the part that actually was. A callout that starts with the open door test usually resolves in a single visit, since the glazier arrives already knowing which of the two faults to expect rather than working through possibilities on site.

It is also worth factoring in that a second wasted cylinder is not just money spent on the wrong part, it is time spent with a door that either will not lock at all or only locks with an insecure workaround, which carries its own risk the longer it continues.

Mistakes That Make This Worse

Forcing the handle repeatedly once it becomes clear the door will not lock is the single most common way a keep alignment issue turns into a genuinely broken gearbox. The mechanism is not designed to be forced past resistance, and repeated attempts can crack internal components that were otherwise still functional.

Buying and fitting a second or third cylinder without first doing the open door test wastes money on a part that was diagnosed incorrectly from the start, and it delays finding the fault that is actually causing the problem.

Ignoring the issue because a workaround has been found, such as always double checking the door with extra force, leaves a genuine security gap in place. A door that only locks with effort is also a door that is easier to force from the outside, since the same misalignment that resists a key also offers less resistance to someone trying to defeat the lock deliberately.

Assuming the whole door needs replacing is the costliest mistake of all, and it is one some homeowners jump to once a cylinder swap has failed to fix things. A gearbox and a set of keeps are both replaceable components within an otherwise sound door, and neither fault on its own is a reason to consider a full replacement.

When It Is Worth Calling Someone Rather Than Testing It Yourself

If the open door test shows the hooks moving unevenly, only partially, or with a grinding or catching sound, that is a sign of internal gearbox damage that is not safe to keep operating, since forcing it further tends to turn a straightforward parts swap into a more awkward extraction job.

If the door has visibly dropped, meaning it catches on the frame when opening or closing even before you try to lock it, a keep adjustment on its own may not be enough, and the hinges are worth having checked at the same visit rather than treating the lock separately from the door’s overall fit.

If you are dealing with either of these situations, our UPVC lock repair service covers exactly this diagnosis and fix, without needing to guess at parts before someone has actually looked at the mechanism itself.

How This Differs From Other Lock Faults We Cover

This particular scenario, a cylinder already replaced with no improvement, is different from the general wear and jamming most homeowners search for first. For the wider picture of how a UPVC lock deteriorates over years of normal use before it reaches this point, our breakdown of why UPVC door locks jam in the first place covers that gradual process in more depth.

If your lock is behaving differently depending on the weather rather than staying consistently faulty, that points toward a separate cause entirely, and the four distinct reasons a UPVC lock can seize up completely walks through ice, dried lubrication, grit and coastal corrosion as their own separate diagnoses, none of which involve the gearbox or keep alignment fault this post is about.

Beyond the lock itself, a door that has dropped enough to throw the keeps out of line sometimes points to a wider issue with the door’s fit in the frame, which our UPVC door repair work across Southampton addresses alongside the lock mechanism where the two faults turn out to be connected.

Frequently Asked Questions

I replaced the cylinder and the door still won’t lock, what should I check next?

Open the door fully and lift the handle while watching the hooks and rollers along the edge. If they move out smoothly on their own, the fault is with the keeps on the frame. If they move poorly or not at all, the gearbox itself has failed and needs replacing.

Can a faulty gearbox be repaired, or does it always need replacing?

A gearbox is a sealed mechanical unit and is almost always replaced as a whole rather than repaired internally, since opening one up rarely restores reliable long term operation. Replacement is usually quicker and more dependable than attempting a partial fix.

Is it possible the new cylinder itself was faulty?

It is possible but uncommon, and the open door test rules this out quickly. If the hooks and rollers move correctly with the door open, the cylinder and the mechanism it controls are both working, and any remaining fault is on the frame side rather than inside the door.

How do I know if my door has dropped enough to need hinge work as well as a keep adjustment?

If the door catches, scrapes, or needs to be lifted slightly to close properly even before you attempt to lock it, that is a sign the door itself has dropped, and a keep adjustment on its own is unlikely to hold for long without the hinges also being checked.

Does this same misdiagnosis happen with window locks, or only doors?

It happens with espagnolette window locks too, though less often, since window handles carry less weight and drop less frequently than a full door. The same open handle test applies, watching whether the locking bar moves fully and smoothly before assuming the handle or cylinder itself is at fault.

Final Thoughts

A door that still won’t lock after a new cylinder is not a mystery, it’s a sign the fault was never in the part that got replaced. The gearbox and the keeps do the actual mechanical work of locking a UPVC door, and either one failing produces symptoms that look identical to a bad cylinder from the outside.

A thirty second open door test tells you which side of the door the real fault is on, before you spend anything further guessing at parts. If that test has pointed you toward a gearbox or a door that has dropped out of alignment, get in touch with our Southampton team and we’ll confirm exactly what needs fixing before any further parts get bought.