The Complete Homeowner Guide to Door Locks, Mechanisms & Common Faults
Introduction
Modern uPVC and composite doors use sophisticated multipoint locking systems designed to provide excellent security, weather sealing and smooth operation. Although they appear simple from the outside, they contain dozens of individual components working together every time you open or lock the door.
When one part begins to wear, it often affects the rest of the system. A dropped door can damage a gearbox, worn handles can be mistaken for a failed mechanism, and poor alignment can eventually destroy an otherwise perfectly good lock.
This guide explains every major component, what it does, how it fails and what can usually be repaired.
A
Active Door
The main opening door on a pair of French doors.
Purpose
Contains the primary multipoint locking mechanism.
- Poor alignment.
- Difficult locking.
- Handle won't lift.
- Hooks not engaging.
Typical repair
- Door alignment.
- Gearbox replacement.
- Keep adjustment.
Automatic Multipoint Lock
Automatically engages when the door closes.
Common faults
- Auto latch fails.
- Lock won't release.
- Internal springs wear.
Typical repair
- Gearbox replacement.
- Spring replacement.
B
Backset
Distance from the faceplate to the centre of the spindle.
Common sizes
25mm • 28mm • 30mm • 35mm • 40mm • 45mm • 50mm
Problems
Wrong backset means the replacement gearbox simply won't fit.
C
Centre Case (Gearbox Housing)
The centre case houses the gearbox and connects all locking points.
Common faults
- Internal wear.
- Broken cam.
- Broken gate.
- Broken follower.
Symptoms
- Handle drops.
- Key turns but nothing happens.
- Door jammed shut.
- Door won't unlock.
Typical repair
Replace gearbox.
Compression Rollers
Adjustable rollers which compress the weather seals.
Problems
- Incorrect adjustment.
- Worn bearings.
- Roller seized.
Symptoms
- Draughts.
- Door rattles.
- Heavy handle.
Repair
Adjustment or replacement.
D
Deadbolt
Solid locking bolt.
Problems
- Doesn't fully engage.
- Binding against keep.
Symptoms
- Key won't fully turn.
- Lock difficult to operate.
E
Euro Cylinder
The removable lock operated by your key.
Available as
- Standard
- Anti Snap
- 1 Star
- 3 Star
- Thumbturn
- Double cylinder
- Half cylinder
Common faults
- Wrong size.
- Worn pins.
- Broken cam.
- Key sticking.
- Key won't remove.
- Key snapped inside.
Symptoms
- Key difficult to turn.
- Lock feels rough.
- Key jams.
Repair
Cylinder replacement.
F
Faceplate
The visible strip running down the edge of the mechanism.
Problems
- Bent.
- Corroded.
- Wrong replacement length.
G
Gearbox
The heart of every multipoint lock.
Controls:
- Latch
- Deadbolt
- Hooks
- Rollers
- Shoot bolts
Most common faults
- Broken spring.
- Broken follower.
- Worn cam.
- Broken gate.
- Internal wear.
Symptoms
- Door jammed shut.
- Handle won't lift.
- Handles drop.
- Key turns but door won't open.
- Latch won't retract.
Typical repair
Replace gearbox.
H
Handles
One of the most commonly replaced parts.
Available as
- Lever Lever
- Lever Pad
- Lever Fixed Pad
- Inline
- Offset
- Sprung
- Unsprung
- Adjustable
Common faults
- Broken springs.
- Loose backplate.
- Drooping handles.
- Cracked casting.
- Worn spindle follower.
Symptoms
- Handle hangs down.
- Doesn't return.
- Loose feel.
- Difficult lifting.
Typical repair
Replace handles.
Hooks
Curved locking points.
Problems
- Seize.
- Break.
- Become stiff.
Symptoms
- Grinding.
- Heavy handle.
- Lock won't engage.
Often cleaned rather than replaced.
K
Keeps
Metal receivers fitted to the frame.
Problems
- Out of alignment.
- Loose.
- Worn.
Symptoms
- Door catches.
- Heavy handle.
- Lock difficult to operate.
Usually adjusted rather than replaced.
L
Latch
Keeps the door closed.
Common faults
- Broken spring.
- Won't retract.
- Sticking.
Symptoms
- Door won't open.
- Door won't close.
M
Multipoint Lock
Complete locking system.
Contains
- Gearbox.
- Hooks.
- Rollers.
- Deadbolt.
- Faceplate.
- Drive bars.
Common faults
- Gearbox failure.
- Hook seizure.
- Roller seizure.
- Alignment issues.
- Obsolete mechanism.
P
Passive Door Lock
Secondary lock fitted to French doors.
Common faults
- Shoot bolts seize.
- Centre column moves.
- Door drops.
R
Rollers
Used to compress seals.
Common faults
- Seized.
- Incorrectly adjusted.
- Worn.
S
Shoot Bolts
Vertical bolts found on French doors.
Problems
- Stick.
- Corrode.
- Won't engage.
Spindle
Square steel bar connecting both handles.
Common faults
- Worn.
- Snapped.
- Wrong size.
Spring Cassette
Returns the handles to horizontal.
Common faults
- Weak springs.
- Broken springs.
Symptoms
- Drooping handles.
Usually replaced with the handles.
T
Top Hook
Upper locking point.
Problems
- Dirt.
- Rust.
- Lack of lubrication.
Often repairable by dismantling and cleaning.
U
Universal Gearbox
Designed to replace multiple original gearboxes.
Advantages
- Widely available.
Disadvantages
- Doesn't always replicate the original mechanism.
- May require modification.
- Sometimes original parts are the better repair.
W
Weather Seals
Rubber seals around the door.
Problems
- Flattened.
- Torn.
- Excessive compression.
Symptoms
- Draughts.
- Water ingress.
Common Fault Symptoms
If your door has any of the following:
- Handle won't lift.
- Handles droop.
- Key won't turn.
- Key difficult to remove.
- Door only locks when lifted.
- Door catches the frame.
- Grinding when locking.
- Door jammed shut.
- Door only locks when open.
- Hooks won't move.
- Rollers don't engage.
- Key turns but nothing happens.
...the fault may be the gearbox, handles, euro cylinder, alignment, hooks or frame keeps rather than the complete locking mechanism.
Most Problems Are Repairable
One of the biggest myths surrounding uPVC and composite doors is that a faulty locking mechanism means a new door is required. In reality, most faults involve only one or two worn components. Gearboxes, handles, euro cylinders, hooks, rollers and even many obsolete mechanisms can often be repaired or replaced while retaining the existing door. Combined with correct door alignment, these repairs restore smooth operation, improve security and can extend the life of the door for many years.
Related article: How much does a door repair cost?
