A UPVC door that won't lock is usually a failed gearbox, misaligned door or seized locking point. This guide explains the five most common causes, how to identify each one, and what the correct repair involves.
Why UPVC Door Locks Fail
UPVC doors use multipoint locking systems — mechanisms with multiple locking points distributed along the door edge, all operated by a single handle. This is more secure than a single-point lock, but it also means there are more components that can fail over time.
Here are the five most common causes of UPVC door locking problems.
1. Failed Gearbox
The gearbox is the central component that converts the movement of the handle into locking bolt movement. When it fails, the most common symptom is a handle that drops rather than springing back into position, or a handle that moves but doesn't drive the locking points.
Gearbox failure is the most common UPVC lock problem and is almost always caused by wear after years of use. **Fix:** Gearbox replacement. This is a job for a locksmith — the mechanism needs to be correctly identified and sourced, then installed precisely.
2. Door Misalignment
Over time, UPVC doors can shift slightly in their frames due to settlement, hinge wear or changes in the frame's position. Even a few millimetres of misalignment can cause the locking bolts to miss their keeps in the frame, preventing the door from locking.
Symptoms include: a door that 'jumps' slightly when you try to lock it, or one that requires you to lift the handle or push the door at a particular angle before it will engage.
**Fix:** Door realignment, which may involve adjusting the hinges, the keep plates, or both. This is usually a straightforward job.
3. Worn or Corroded Locking Points
The hook bolts, rollers and mushroom bolts that protrude from the door edge can wear, corrode, or get jammed with debris over time. A single seized locking point can prevent the whole multipoint mechanism from operating.
**Fix:** Lubrication with a suitable UPVC lubricant (never WD-40) can resolve mild cases. Severely worn or corroded points may need individual replacement.
4. Cylinder Problem
If the handle is working normally and the locking points seem fine, but you can't turn the key to lock the door, the issue may be with the cylinder rather than the mechanism. Cylinders can fail due to wear, damage, or (in the case of attempted break-ins) from a failed snapping attempt.
**Fix:** Cylinder replacement. This is a good time to upgrade to an anti-snap cylinder.
5. Damaged or Dropped Keep Plates
The keep plates are the metal plates set into the door frame that the locking points engage with. If a keep has been knocked out of alignment (by door impact or frame movement), the bolt won't engage properly.
**Fix:** Realigning or replacing the keep plates, which is a relatively simple repair.
What You Should (and Shouldn't) Do
**Do:** Call a locksmith for diagnosis and repair. These mechanisms can be difficult to identify without experience and the wrong part will simply waste money.
**Don't:** Keep forcing the handle repeatedly. This accelerates gearbox wear and can cause additional damage.
**Don't:** Use WD-40 to lubricate a UPVC mechanism. It's a solvent that can damage the plastic components and will attract more dirt over time.
**Do:** Lubricate your UPVC door annually with a dedicated UPVC lubricant or light machine oil applied carefully to the locking points.
If your UPVC door is playing up, we're happy to come and take a look. Most repairs are completed in a single visit.