Your uPVC door lock isn’t working. Something has clearly failed. But what?
Getting the diagnosis right matters—cylinder replacement is a fraction of the cost of gearbox work. This guide helps you identify which component has actually failed before you call for help. For more information, see our detailed multipoint locks explained guide.
Understanding the Two Components
The Cylinder
Located in the centre of the door edge, where you insert your key.
Its job:
- Accept and recognise your key
- Allow rotation when correct key is used
- Engage a cam that locks/unlocks the gearbox
What can fail:
- Internal pins wear or break
- Cam (tailpiece) snaps
- Cylinder seizes from corrosion or debris
- Wrong size prevents engagement—see our sizing guide
The Gearbox
Located behind the handles, inside the door.
Its job:
- Convert handle lift into hook/bolt movement
- Provide spring return for handle
- Connect to cylinder cam for locking
What can fail:
- Internal springs break
- Gears strip or seize
- Spindle hole wears out
- Internal mechanism jams
For detailed troubleshooting, see uPVC multipoint lock problems.
"The cylinder and gearbox work together but are separate components. Accurate diagnosis means identifying which one has actually failed—not just guessing.
Diagnostic Test 1: The Key Turn Test
What to Do
With the door closed normally:
- Insert your key fully
- Attempt to turn it
- Note exactly what happens
Interpreting Results
| What Happens | Likely Problem |
|---|---|
| Key won’t turn at all | Cylinder (pins) or alignment |
| Key turns smoothly but nothing happens | Cylinder (cam) or gearbox |
| Key turns with resistance, nothing happens | Gearbox |
| Key turns, lock operates | Neither—likely alignment |
Key Insight
If the key has no resistance when turning but the lock doesn’t operate, the cylinder cam is probably broken or the cylinder is too short. If there’s resistance but no result, the gearbox isn’t responding.
Diagnostic Test 2: The Open Door Test
What to Do
Open the door fully (propped open), then:
- Lift the handle—does it move smoothly and spring back?
- Turn the key—does it turn with normal resistance?
- Watch the locking points—do they move when handle lifts?
Interpreting Results
Handle problem (floppy, stiff, no spring): → Gearbox issue (internal spring or spindle)
Handle works, locking points don’t move: → Gearbox issue (internal mechanism)
Handle works, points move, key spins freely: → Cylinder issue (cam broken)
Everything works with door open: → Alignment issue (not cylinder or gearbox)
Diagnostic Test 3: The Handle Lift Test
What to Do
With door open, focus on the handle:
- Lift it slowly and release
- Note smoothness and spring return
- Listen for unusual sounds
Interpreting Results
| Observation | Indicates |
|---|---|
| Smooth lift, springs back | Gearbox OK |
| Stiff or grinding | Gearbox wear/damage |
| Floppy, no resistance | Gearbox spring broken |
| Lift works but points don’t move | Gearbox internal failure |
Handle vs Spindle
The handle itself can fail (broken spring inside handle) separately from the gearbox. If you’re unsure, remove the handles and try operating the gearbox spindle directly with a screwdriver.
Diagnostic Test 4: The Key Resistance Test
What to Do
With door open and handle lifted to the “locked” position:
- Turn the key slowly
- Feel the resistance carefully
- Note if it “catches” or spins freely
Interpreting Results
Normal resistance, then locks: → Cylinder working correctly
Spins freely with no resistance: → Cylinder cam broken or not engaging gearbox
Heavy resistance, won’t complete turn: → Gearbox not accepting cylinder engagement
Turn requires excessive force: → Cylinder pins worn, or gearbox binding
Common Misdiagnoses
Misdiagnosis 1: “Gearbox Failed” When Cylinder is Wrong Size
Symptoms: Key turns, nothing happens Actual cause: Cylinder too short, cam doesn’t reach gearbox Correct fix: Replace cylinder with correct length
Misdiagnosis 2: “Cylinder Failed” When Alignment is Off
Symptoms: Key won’t turn when door closed Actual cause: Door misalignment puts pressure on mechanism Correct fix: Hinge adjustment, not cylinder replacement
Misdiagnosis 3: “Need New Lock” When Only Cam Broke
Symptoms: Key spins freely Actual cause: Cylinder cam (tailpiece) snapped Correct fix: Just replace cylinder (£60-120), not whole mechanism (£200+). See our guide on when to repair vs replace locks.
Cost Implications
Getting diagnosis right saves significant money:
| Component | Typical Cost | Repair Time |
|---|---|---|
| Cylinder replacement | £60-120 | 15-20 min |
| Gearbox only | £150-250 | 30-45 min |
| Full mechanism | £250-400 | 60-90 min |
| Hinge adjustment | £40-80 | 20-30 min |
"A broken cam looks like a catastrophic failure but costs under £100 to fix. A seized gearbox diagnosed as “just needs a new cylinder” wastes money and doesn’t solve the problem.
When You Need Professional Diagnosis
DIY Diagnosis Works When:
- You can access the door open and closed
- The symptoms are clear
- You can perform the tests described above
Professional Diagnosis Required When:
- You’re locked out (can’t open door)
- Multiple symptoms present
- Tests give contradictory results
- You’re not confident in your assessment
Communicating with Your Locksmith
Armed with your diagnostic information:
Tell them:
- “Key turns freely with no resistance”
- “Handle won’t lift, no spring return”
- “Works with door open, not closed”
Not:
- “Lock is broken”
- “I think I need a new mechanism”
- “Something’s wrong with the door”
Specific symptoms help locksmiths arrive with correct parts and give accurate quotes.
Frequently Asked Questions
What's cheaper to fix—cylinder or gearbox?
Cylinders are significantly cheaper. Cylinder replacement typically costs £60-120. Gearbox replacement costs £150-300 including labour. Getting the diagnosis right saves money.
Can I check myself before calling a locksmith?
Yes. The tests in this guide require no special tools. Knowing whether it's the cylinder or gearbox helps you understand the repair quote and confirms the locksmith's diagnosis.
Can both fail at the same time?
Possible but uncommon. Usually one fails first. However, a seized gearbox can damage a cylinder over time (or vice versa). A good locksmith checks both.
If it's the gearbox, do I need a whole new mechanism?
Sometimes just the gearbox can be replaced if the rest of the mechanism (hooks, bolts, connecting bars) is sound. A locksmith can assess whether partial replacement is viable.
Can a cylinder cause gearbox-like symptoms?
Yes, if the cylinder cam is broken or the cylinder is the wrong length. The cylinder may turn but not engage the gearbox—mimicking gearbox failure. Always check both.
Should I try to remove the cylinder myself?
If you can access the fixing screw (door must be open), cylinder removal is simple. This can help diagnose by eliminating the cylinder from the equation.
How long do gearboxes typically last before needing replacement?
Quality gearboxes last 15-20 years with normal use. Heavy use, poor maintenance, or misalignment can shorten lifespan. Signs of wear often appear gradually before complete failure.
Is it worth repairing an old gearbox or should I replace the whole mechanism?
If the mechanism is over 15 years old and showing problems, replacing the complete unit often provides better value. Repairing one component may lead to others failing soon after.
Take Action
Before calling for repair:
- Perform the tests described above
- Note specific symptoms (not just “it’s broken”)
- Document what works and what doesn’t
- Share this information when requesting quotes
Accurate diagnosis saves money, speeds repair, and ensures you’re paying for the fix you actually need.