Why won't my uPVC door lock when it's hot?

uPVC expands in high temperatures - even 1-2mm of thermal expansion can shift the door enough that the multipoint hooks and rollers no longer align with the keeps in the frame. Try gently lifting the door handle while pushing the door slightly inward to see if the locking points engage. If the problem only appears in warm weather and the door works fine in winter, the hinges may need adjusting to compensate for the seasonal movement. A locksmith can realign the door so it locks reliably year-round.

Why is my uPVC door handle stiff and hard to lift?

A stiff handle that is hard to lift usually means one of three things: the door has dropped or shifted out of alignment so the locking points are binding against the keeps; the multipoint gearbox is wearing out and losing its spring return; or the mechanism needs lubrication. Test by trying to lift the handle with the door open - if it moves freely then but not when shut, misalignment is the cause. If it is stiff in both positions, the gearbox is the more likely culprit and will need replacing.

What is a uPVC multipoint gearbox and why does it fail?

The gearbox is the central component behind the handle that co-ordinates all the locking points - hooks, rollers, and bolts - when you lift the handle and turn the key. Over time, the internal springs and cams wear down from repeated use, causing grinding, a loose handle, or a handle that lifts but engages nothing. Once a gearbox is worn or broken it cannot be repaired - it must be replaced. A locksmith can match and fit a compatible replacement gearbox, usually within a single visit.

Can I fix a uPVC door that won't lock myself?

Yes, for minor issues. Lubrication with a PTFE or silicone spray and minor hinge adjustment are straightforward DIY tasks that fix many alignment and stiffness problems. However, if the gearbox is worn, a locking point has snapped, or the euro cylinder is faulty, those parts need replacing - work best left to a locksmith to avoid damaging the door or leaving it insecure.

How do I know if my uPVC door lock problem is the gearbox or the cylinder?

Lift the door handle with the door open. If the hooks and rollers move up and down smoothly, the gearbox is intact - the problem is likely the cylinder or misalignment. If the handle is loose, grinds, or moves without moving the locking points, the gearbox has failed. A cylinder fault usually presents as difficulty inserting or turning the key, or the key spinning freely without engaging the latch.

Why does my uPVC door latch but the key won't turn to deadlock it?

If the door latches on closing but the key will not turn to throw the deadbolts and hooks, the most common causes are a worn or faulty euro cylinder, a misaligned door where the hooks cannot reach their keeps, or a gearbox that has seized. Try lubricating the cylinder keyway with a dry graphite or PTFE spray first. If the key still will not turn or the cylinder feels loose and sloppy, cylinder replacement is usually the correct fix.

How much does it cost to fix a uPVC door that won't lock in London?

The cost depends on the fault. City Locksmith London charges from £79 for emergency door opening if you are locked out, from £69 labour for a lock change (cylinder replacement), and from £99 to repair or replace a faulty multipoint mechanism or gearbox. All prices exclude VAT at 20%, there is no call-out fee, and a free quote is provided before any work begins.

Why Your uPVC Door Won't Lock (and How to Fix It)

A uPVC door that won’t lock is almost always caused by one of four faults: a dropped or thermally expanded door that has shifted out of alignment, a worn multipoint gearbox behind the handle, a faulty euro cylinder, or locking points that are no longer reaching their keeps in the frame. Before calling a locksmith, try lifting the handle firmly while pushing the door gently inward - if the hooks then engage, misalignment is the culprit and hinge adjustment will usually solve it.

What Actually Happens When You Lock a uPVC Door

Understanding the mechanism helps you diagnose the fault quickly. A uPVC door uses a multipoint locking system rather than a simple single-point latch. When you lift the door handle, a gearbox inside the door body drives multiple locking points - typically a latch, two or more hooks, and one or more roller cams - upward along a strip that runs the full height of the door. Turning the key then deadlocks everything in place.

Every one of those locking points must align precisely with a matching keep (also called a strike plate) on the door frame. If the door has moved even a millimetre or two out of position, or the gearbox can no longer drive the points home, the system fails.

Common Causes of a uPVC Door That Won’t Lock

1. Door Misalignment - Dropped Hinges or Thermal Expansion

This is the single most frequent cause. uPVC doors are heavy and the hinges bear that load every time the door swings. Over months and years, hinge screws loosen and the door drops fractionally. The locking points no longer meet their keeps squarely.

Thermal expansion compounds the problem. uPVC expands in warm weather - even 1-2mm of movement is enough to prevent the hooks from seating. Many homeowners notice the door locks fine in winter but fights them on hot summer days.

How to spot it: The door works fine when you test it with it held slightly open. Once it is fully closed against the frame, the handle is stiff or the key will not turn. Alternatively, you notice the door scraping at the bottom or along one edge.

2. Worn or Broken Multipoint Gearbox

The gearbox is the mechanical brain of the multipoint system. It converts the up-and-down movement of the handle into the motion that drives all the locking points simultaneously. Internal springs and cams wear down with repeated use.

Early signs include a grinding or crunching feeling when you lift the handle, a handle that no longer springs back, or a handle that lifts but moves nothing. At the end-stage, the gearbox breaks internally and the door either cannot be locked at all or cannot be opened.

How to spot it: The handle is stiff or grinding whether the door is open or closed. This rules out misalignment (which only causes stiffness when shut) and points to a mechanical failure inside the gearbox itself.

3. Faulty Euro Cylinder

The euro cylinder houses the keyway and drives the latch. A cylinder that is worn, has been damaged in an attempted break-in, or is simply old may allow the key to turn without engaging anything, or may prevent the key from turning at all.

Cylinder problems also arise when someone installs a cylinder that is the wrong size and it protrudes beyond the handle escutcheon - this creates a snapping risk and can interfere with the locking action.

How to spot it: The handle lifts normally and the locking points move, but the key will not turn, turns without resistance (spinning freely), or is very hard to insert and remove.

4. Locking Points Not Reaching the Keeps

Even when the gearbox and cylinder are working correctly, the locking points must have clear, correctly positioned keeps to engage with. Keeps can work loose from the frame over time, shift slightly, or corrode. One stubborn keep that sits a millimetre too far out can prevent the whole system from deadlocking.

How to spot it: You can see or feel one specific point on the door edge where resistance occurs. Pushing the door inward at that point while turning the key sometimes helps the hook seat - confirming the keep needs adjusting.

Causes and Fixes at a Glance

Fault	Key symptom	DIY fix	When to call a locksmith
Door misalignment	Stiff only when shut; scraping edge	Tighten hinge screws; adjust flag hinges with Allen key	If hinges are corroded, broken, or the door has old butt hinges with no adjuster
Thermal expansion	Fine in winter, fails in heat	Lift handle firmly and push door inward while locking	If adjustment does not correct it seasonally
Worn gearbox	Grinding/loose handle; stiff open and shut	Lubricate with PTFE spray as a temporary measure	Always - gearbox replacement is not a DIY job
Faulty euro cylinder	Key won’t turn or spins freely	Try PTFE spray in keyway	If lubrication does not resolve it - cylinder replacement from £69 labour
Loose or shifted keep	Resistance at one specific point	Loosen keep screws, nudge into alignment, re-tighten	If the keep is damaged or the frame has moved significantly

DIY Checks to Try First

Work through these steps in order before arranging a callout. Many uPVC locking problems clear without any specialist tools.

Test with the door open. Lift the handle and watch all the locking points move. If they move freely and spring back correctly, the gearbox is intact. The problem is likely alignment or the cylinder.
Lubricate the mechanism. Spray a PTFE or dry silicone lubricant onto the exposed locking points, hooks, and roller cams along the door edge. Work the handle several times to distribute the lubricant. Avoid oil-based sprays - they attract grit. Do not spray lubricant inside the euro cylinder keyway - use a dry graphite spray for the keyway only.
Check and tighten hinge screws. Open the door fully and inspect all hinges. Tighten any loose screws. A quarter-turn is often enough to lift the door back into square.
Adjust the flag hinges. Most uPVC doors fitted after 2000 have adjustable flag hinges with Allen key sockets. The vertical adjuster moves the door up or down; the lateral adjuster moves it toward or away from the frame. Small adjustments - half a turn at a time - can bring a dropped door back into alignment.
Try the lift-and-push technique. With the door closed, lift the handle firmly upward while pushing the door slightly inward toward the frame. If the lock then engages, the locking points are just short of their keeps - hinge adjustment or keep repositioning will cure it permanently.
Inspect and adjust the keeps. With a torch, check that each keep on the frame is flush, undamaged, and its screws are tight. Loosen the screws slightly, push the keep to where the locking point naturally wants to go, then re-tighten.
Test the cylinder separately. With the door open and the handle down (locking points retracted), insert the key and turn it. It should move smoothly. If it is stiff, sticky, or turns without resistance, the cylinder is the fault.

When It Is the Gearbox - and Why DIY Will Not Fix It

If the handle grinds, feels loose with no spring return, or moves without driving the locking points, the gearbox has failed mechanically. Unlike a cylinder, a gearbox cannot be lubricated back to health once the internal components have worn through or snapped.

Replacing a gearbox requires removing the door handle and panel, sliding out the multipoint strip, and fitting a compatible replacement - which means measuring the backset (distance from door edge to handle centre), the PZ measurement (centre-to-centre distance between handle spindle and cylinder), and the overall strip length. Getting these wrong means ordering the wrong part, which is why most homeowners prefer a locksmith to handle the measurement and fitting in a single visit.

City Locksmith London’s uPVC locksmith team carries common gearbox sizes and can usually replace a failed mechanism within a single callout, with mechanism repair and replacement from £99 (ex VAT).

When It Is the Cylinder - Replacement or Upgrade

A euro cylinder that is worn, damaged, or the wrong size for the lock body should be replaced rather than forced. Replacement is also the right moment to upgrade to a TS007 3-star anti-snap cylinder from a brand such as Ultion, Brisant, or Mul-T-Lock - lock snapping remains a common burglary method on uPVC doors across London.

For step-by-step guidance on measuring and fitting a replacement cylinder yourself, see our detailed guide: How to Replace a Barrel Lock in Your uPVC Door.

If you would rather have a professional supply and fit the cylinder, our lock change service covers cylinder replacement from £69 labour (ex VAT), with no call-out fee and a free quote before we start.

What If the Door Is Locked and Won’t Open?

If the door is locked and the handle or key no longer operate it at all, you need emergency access. Forcing a uPVC door without the right technique causes significant frame and mechanism damage. City Locksmith London provides emergency locksmith cover across London with a 25-minute response, available 24 hours a day, 365 days a year. Emergency door opening starts from £79 (ex VAT) and we will not cause unnecessary damage to your door or frame.

Preventative Maintenance to Avoid Future Problems

A small amount of routine care extends the life of a uPVC multipoint system considerably.

Lubricate once or twice a year - PTFE or silicone spray on the locking points and strip, graphite spray in the keyway.
Check hinge screws each spring and autumn - thermal cycling gradually works screws loose.
Do not lift the handle hard from the outside - operating the handle correctly (always from the inside pull, or by gripping the handle properly from outside) reduces gearbox wear.
Address stiffness early - a door that requires extra force to lock is putting excessive load on the gearbox. Fixing the alignment issue now avoids a gearbox replacement later.

Need a Locksmith for Your uPVC Door?

City Locksmith London covers all London postcodes, 24/7, with no call-out fee and fully insured, DBS-checked engineers. We carry parts for most common uPVC multipoint systems and can diagnose and fix the majority of locking faults in a single visit. For current pricing on mechanism repair, cylinder replacement, and emergency callouts, see our locksmith price list.

Contact us for a free quote, or if you are locked out right now, call us for emergency assistance - our emergency locksmith service covers Greater London with a 25-minute response. For non-urgent lock upgrades, you can also explore our door lock installation service if a full lock replacement is the best solution for your door.