Key Worked This Morning, Now Won't Turn: Translation Decision Tree

Why this matters

"It worked this morning and now it won't turn" is the single most common intermittent-lock call, and it is almost always a clue, not a mystery. A lock that fails suddenly between two normal uses rarely has a worn cylinder or a worn key as the root cause, because wear is gradual. Something changed in the last few hours: temperature, a moved door, a dropped key, or a slug of debris that finally bridged a chamber. Translating the customer's timeline into a mechanical cause before you touch the cylinder saves you from tearing down a lock that was fine and pinning a wear problem that was actually a thermal one.

Symptom presentation

Get the timeline precisely. Ask three questions before you handle anything:

• "When did it last work, and what is different now?" Morning-to-evening points to thermal expansion or a sun-loaded door. Worked-yesterday-failed-today points to overnight settling or moisture.
• "Does the key go all the way in?" Full insertion that won't rotate is a pinning or plug problem. Partial insertion is a debris, broken-key-tip, or wrong-key issue.
• "Did you drop the key, lend it, or copy it recently?" A fresh duplicate or a bent key reframes the whole call.

Have the customer demonstrate if they are present. Watch whether the key seats fully and where in the rotation it stops.

Quick checks

Run these before any teardown. They cost two minutes and resolve a large share of these calls:

• Try the customer's key, then a known-good original if one exists. A duplicate that binds while the original works is a key-cut problem, not a lock problem.
• Insert the key fully and apply light rotational pressure while pushing in and pulling out a hair. If it frees on the pull, a pin is hanging at the shear line from debris or a high cut.
• Check door movement. Lift the door by the knob or push/pull the leaf. If the bolt or latch frees when you take weight off the door, the lock is fine and the door has moved.
• Feel the key temperature and the door's sun exposure. A south or west metal door at midday can grow enough to bind a deadbolt that was free at dawn.

Isolation tree

Branch 1: Key goes fully in, won't rotate, original key also fails. The cylinder changed. Spray a non-gumming PTFE or graphite-rated lock lubricant, work the plug, and retest. If it frees, a sticky or debris-bridged pin was the cause. If it still binds with no door load, pull the cylinder and inspect for a sheared spring or a pin that migrated.

Branch 2: Original key works, duplicate does not. The duplicate is the fault. Compare cut depths with a gauge or against the original. A single shallow cut leaves a pin proud of the shear line. Re-cut or hand-dress the offending cut.

Branch 3: Key binds only under door load, frees when door weight is lifted. This is a strike or alignment problem, not the cylinder. The bolt is dragging the strike lip or the door has sagged. Jump to the door/strike branch.

Branch 4: Key will not seat fully. Look down the keyway with a light. A broken key tip, a foreign object, or a frost plug blocks insertion. Extract the obstruction before any rotation diagnosis.

Branch 5: Worked warm, fails hot or cold. Thermal. Metal doors and bolts expand into a tight strike pocket in heat; lubricant thickens and pins drag in cold. Confirm by testing at a different temperature or shading the door.

Branch 6: Fresh moisture or a recent rain, lock stiff this morning. Water intrusion into the cylinder followed by overnight cooling can frost or gum the pin chambers, especially on an exposed exterior door with no weatherhead. The lock that worked yesterday afternoon seizes at dawn and frees once the sun warms it. Confirm by warming the cylinder gently and re-testing; if it frees, address water intrusion and switch to a moisture-tolerant dry lubricant.

A diagnostic discipline worth stating: on a sudden-onset complaint, resist the urge to immediately pull and rebuild the cylinder. Sudden failure between two normal uses almost never means a worn cylinder, because wear is gradual. The fast money is on the recent change, the duplicate key, the moved door, the temperature, or the slug of debris that finally bridged a chamber. Chase the change first.

Confirming diagnosis

Confirm before you cut metal. For a cylinder fault, the plug should rotate freely with the cylinder out of the door and a good key inserted; if it binds out of the door, the fault is internal. For an alignment fault, the bolt should throw cleanly with the door open and bind only when closed. For a key fault, a freshly cut correct key should rotate with the same light pressure the original needs. Never escalate to drilling on an intermittent call until you have ruled out door load and a sticky pin, both of which masquerade as a seized cylinder.

Remediation

• Sticky or debris-bridged pin: flush with lock lubricant, cycle, and if it recurs, pull and clean the cylinder.
• Bad duplicate: re-cut to depth or replace the blank.
• Door sag or strike drag: shim the hinge, adjust the strike, or relieve the strike mouth so the bolt clears under load.
• Thermal bind: increase strike clearance and switch to a low-temperature-stable PTFE lubricant.
• Sheared spring or migrated pin: rebuild the cylinder with fresh springs and pins to spec.

Document the temperature, door exposure, and which key was used. An intermittent that you "fixed" with lubricant will come back if the real cause was the door.

References

• ANSI/BHMA A156.5, American National Standard for Cylinders and Input Devices for Locks.
• ANSI/BHMA A156.2, Bored and Preassembled Locks and Latches, operational and cycle requirements.
• NFPA 101, Life Safety Code, egress door operability requirements for hardware that must remain operable.