Mechanical locks: the basics
A commercial mechanical lock has three components. The cylinder is the cylinder the key turns — usually a Euro profile (DIN 70 mm) in commercial UK use. The lock body is the case mortised into the door edge that contains the latch, deadbolt, hookbolt or hookbolt-plus-latch mechanism. The strike is the metal plate on the frame that the bolt engages into.
Failure happens in all three, but in different ways and at different rates.
Common mechanical lock faults
The lock body is the most failure-prone of the three. A failed lock body can:
- Refuse to retract the bolt when the key turns (worn lift cam, sticking spindle).
- Refuse to extend the bolt when the door is closed (broken spring, jammed bolt).
- Allow the key to turn but the bolt feels disconnected (broken internal lever).
- Feel stiff and require excess force (case wear, internal corrosion, lack of lubrication).
Electronic access control: where it fails
Access control systems add a layer of electronics over a mechanical lock. The most common types in UK commercial use:
- Magnetic locks (mag-locks) — electromagnet holds the door closed. Releases on power loss (fail-safe) or stays locked on power loss (fail-secure, less common for mag-locks).
- Electric strikes — strike plate on the frame releases electronically while the mechanical lock body stays passive.
- Motorised mortice locks — full lock body with a motor that drives the bolt; key override remains on most.
- Multi-point electric locks — common on hotel and security entrances; integrates locking, exit device and access control in one assembly.
- Plus the controller (the brain of the system), the readers (card, fob, biometric, keypad), the power supply, and the wiring between all of them.
Diagnostic first questions
When the door is not working, the first questions narrow the problem fast. Does the key still operate the lock mechanically? If yes, the lock is fine and the fault is electronic. If no, the lock body is the problem and electronic diagnostics can wait.
If electronic: does the reader respond (LED, beep) when a valid credential is presented? If no, the reader, its power, or its wiring is at fault. If yes (reader responds but door does not unlock), the controller, the lock’s power supply, or the lock itself.
These two questions resolve most callouts to a specific subsystem in under five minutes. The repair from there is targeted.
Common access control faults
Across the field, a handful of faults appear repeatedly.
Mag-lock not releasing — usually the request-to-exit (REX) button or sensor has failed, less commonly the controller’s output relay. Mag-lock itself rarely fails in service.
Reader unresponsive — most often a wiring issue at the reader (water ingress, broken cable), occasionally the reader itself. Bench-test by swapping with a known-good reader if possible.
Some cards work, others do not — controller database or permissions issue, not a hardware fault. Re-issue the affected cards or check user permissions.
Controller offline — power loss, communications loss to head-end (for networked systems), or a hardware failure on the controller board. Reset is the first step; replacement if reset doesn’t hold.
Door drops out intermittently — usually a marginal power supply struggling under summer or winter ambient temperature changes. Power supply replacement is a relatively cheap fix that resolves a frustrating intermittent fault.
Re-key, re-program, replace
Three different jobs that sometimes get conflated.
Re-key — change the lock cylinder so old keys no longer work. Used after staff turnover, lost keys, suspected key copying. 5–15 minutes per lock, low cost.
Re-program — update the access control database to invalidate old credentials and issue new ones. Used after staff turnover on electronic systems. 30 minutes for a typical system, plus credential issue time.
Lock replacement — physically replace the lock body, cylinder or both, usually because the lock has failed mechanically. 30–90 minutes depending on the lock type.
In our experience around half of “we need new locks” callouts are actually re-key requests — and the cost difference is significant. Worth being clear with the contractor about what you actually need.
Maintenance for locks and access control
Mechanical locks benefit from annual lubrication (a graphite-based lock lubricant, not WD-40 which dries to a residue and worsens long-term operation). Cylinders should be cycled with the key a few times per service visit to keep the wafers and pins moving freely.
Electronic access control benefits from annual inspection of all readers (cable terminations, weatherproofing), power supplies (battery condition on backed-up systems, voltage at the door), and controllers (firmware updates, log inspection for repeated denied-access events that might signal a fault).