When your key turns but the lock doesn't actually do anything — the door stays stuck or swings open without resistance — the lock's internal mechanism has almost certainly failed. The good news is that this is a common problem with a handful of well-known causes, and many homeowners can diagnose and fix it themselves with basic tools. Here's how to work through it step by step.
Why Does This Happen?
A lock that spins freely without engaging usually means one of the following:
- A broken cam or tailpiece — the small plastic or metal piece that connects the cylinder to the bolt mechanism has snapped.
- A stripped or worn spindle — the square bar connecting the two handles/knobs has worn down and no longer turns the latch.
- A disconnected or broken latch mechanism — the internal workings of the latch body have failed.
- A loose or detached cylinder — the lock cylinder has come free from the lock body and is spinning independently.
- Worn internal gears — common in older mortice locks or lever-set locks that have had years of use.
Identifying which part has failed will tell you whether this is a DIY fix or a job for a professional locksmith.
Step 1: Identify Your Lock Type
Before pulling anything apart, figure out what kind of lock you have:
| Lock Type | Common Location | Typical Failure |
|---|---|---|
| Tubular latch | Interior doors | Broken spindle or cam |
| Deadbolt (deadlock) | Front/back doors | Broken cam or tailpiece |
| Mortice lock | Older exterior doors | Worn levers or broken cam |
| Rim lock | Older NZ homes | Broken cam or loose cylinder |
| Deadlatch (nightlatch) | Apartment entry doors | Broken tailpiece |
NZ homes — particularly villas and bungalows — often have older mortice locks that can be harder to source parts for. Newer homes typically use tubular latches or deadbolts.
Step 2: Check the Obvious First
Before dismantling anything, do a quick visual check:
- Is the door actually latched? Sometimes the latch bolt is stuck in the retracted position and the door isn't latching at all — the lock works fine, but the latch needs attention.
- Is the cylinder loose? Grab the key and try to wiggle the cylinder. If it moves around, the retaining screw has likely come loose.
- Is the handle or knob loose? A loose handle often means the spindle has slipped out of position.
Step 3: Remove the Lock and Inspect
For a tubular latch or deadbolt:
- Remove the door handle or knob — usually held by a small grub screw on the side of the rose plate, or visible screws on the face plate.
- Slide out the spindle (the square bar between the two handles).
- Unscrew the face plate on the edge of the door and slide out the latch body.
- Inspect the cam (a small plastic or metal tab on the back of the cylinder) — if it's snapped off, that's your culprit.
- Check the spindle for rounding or wear.
For a mortice lock:
- Remove the handle furniture (screws on the rose or backplate).
- Unscrew the cylinder retaining screw from the edge of the door.
- Slide the cylinder out and inspect the cam/tailpiece.
- The mortice lock body itself may need to be removed from the door pocket — this involves unscrewing the faceplate and pulling the lock body out sideways.
Step 4: Replace the Faulty Part
- Broken cam or tailpiece: These are inexpensive and often available at Bunnings or Mitre 10. Match the size and profile carefully — take the old part with you.
- Worn spindle: Spindles come in standard lengths and can be cut to size. Again, match the square dimension (typically 8 mm for most NZ residential locks).
- Failed latch body: If the latch mechanism itself is broken internally, it's usually easier and more cost-effective to replace the entire latch unit.
- Older mortice lock parts: These can be harder to source. A locksmith may need to fabricate or source a replacement — check with a local professional found on tradedirectory.co.nz.
Step 5: Reassemble and Test
Reassemble in reverse order. Before closing the door:
- Test the latch or bolt movement by hand.
- Insert the key and check the full range of motion.
- Check that the bolt or latch aligns correctly with the strike plate on the door frame. If it doesn't, you may need to adjust the strike plate position slightly.
Preventing Future Problems
- Lubricate your locks annually with a dry graphite spray or a purpose-made lock lubricant — avoid WD-40 as it can attract dust and gum up the mechanism over time.
- Don't force a stiff key — this is often what breaks the cam in the first place.
- Check door alignment — a door that has dropped or swollen (common in NZ's humid climates) puts extra stress on the latch and lock mechanism.
When to Call a Professional
Call a qualified locksmith if:
- You can't identify the lock type or source replacement parts.
- The lock is on an exterior door and security is compromised — don't leave your home unsecured overnight.
- The lock is a high-security or insurance-specified deadlock — incorrect reassembly could void your home insurance cover.
- You're dealing with a master key system in a rental or body corporate property.
- The door frame or door itself is damaged and needs repair alongside the lock.
Licensed locksmiths in NZ are not regulated under the same licensing framework as electricians or plumbers, but reputable professionals will carry appropriate insurance and trade qualifications. You can find qualified local locksmiths on tradedirectory.co.nz.
Safety note: If you suspect your lock has been tampered with or forced, treat it as a security incident. Don't rely on a compromised lock — replace it promptly and consider contacting Police if you believe there has been an attempted break-in.
