A lock that works most of the time is harder to diagnose than a lock that never works, because every successful turn tempts you to believe the problem went away. It did not. Intermittent failure is the lock telling you that something inside it, on the key, or in the door around it is sitting right at the edge of its tolerance, and small shifts in temperature, moisture, wear, or the angle of your wrist are enough to push it over that edge. The useful part is that intermittent problems leave patterns, and the patterns point to causes. This page walks through those patterns as a series of branches. Answer each question about your own lock, follow the branch that matches, and by the end you will know whether you are looking at a key problem, a cylinder problem, a door problem, or something that needs a professional to open up.
Start here: does a different key change anything?
Before anything else, find every key that is supposed to operate this lock. If you only have one, the first branch below is still the one to read, because it tells you something either way.
If a second key works every single time while the original key fails now and then, the problem is on the original key, and you can stop suspecting the lock. Keys wear. Every insertion drags the blade across the pins, and over years the cut depths round over and the shoulder that seats the key at the right depth flattens a little. A key that is slightly worn will still lift the pins to the shear line on a good day, when the cylinder is warm and the door is hanging true, and will miss by a hair on a bad day. The fix is not to copy the worn key, which reproduces the wear. Have a fresh key cut from the working spare or, better, from the lock’s code, and retire the worn one.
If both keys fail at about the same rate, the problem is downstream of the key, and you should move to the next branch. Keep the better spare somewhere safe from now on, because a key that has spent years in a drawer is close to its original shape and is the most useful reference you have.
If you only have one key and it fails intermittently, look at it under a good light. Sight down the blade from the tip for a twist or a bend. Check whether the cuts still have crisp flat bottoms or have gone rounded and shiny. A key that has been used to open boxes or pry lids carries a bend you cannot feel in your hand but the cylinder feels every time. If the key looks tired, get one cut by a locksmith who can read the code off the lock or decode the key rather than trace it, then test again before doing anything else.
If the key is fine, then ask when it fails
Intermittent locks almost always fail on a schedule, even if you have not noticed it yet. Spend a week paying attention and a pattern will usually appear, and the most common one involves weather.
If the lock fails in the morning and works by afternoon, or fails after rain and works after a few dry days, the door and frame are moving, not the lock. Wood swells with moisture, and a door that swells a few thousandths against the frame shifts the position of the bolt relative to the strike. On a deadbolt, that shift means the bolt drags against the edge of the strike opening when you try to throw it, and the extra load transfers back through the cylinder to the key. The key feels stiff or refuses to turn, and people naturally blame the cylinder. Test this by opening the door and turning the key with the bolt in free air. If it turns smoothly with the door open and binds with the door closed, the cylinder is innocent and the alignment is the issue. Our page on door lock frozen or stuck covers the alignment side in more depth, and the branch on alignment later on this page picks it up as well.
If it fails with the door closed but not open, then the door is the patient
This is the single most useful test on the page and the one most people skip. Open the door fully. Insert the key and throw the bolt or turn the latch with nothing in the way. Do it a dozen times.
If it works perfectly every time with the door open, the lock itself is healthy and the problem is that the bolt or latch is meeting resistance in the frame. Look at the strike plate. A bolt that has been dragging leaves a bright, polished arc on the edge of the strike opening where it scrapes on the way through. Look at the hinges. A door that has sagged on loose or worn hinge screws drops the latch side by a small amount, and that amount can be enough to make the bolt hit the bottom edge of the strike instead of passing cleanly into the pocket. Look at the weatherstripping. Thick or newly replaced weatherstrip pushes the door outward and moves the bolt away from the strike, so the bolt catches on the lip.
If the door open test fails just as often as the door closed test, the problem lives in the cylinder or the lock body, and you should skip ahead to the wear and debris branches.
For an alignment problem, the fixes range from replacing hinge screws with longer ones that bite into the stud, to filing the strike opening a little larger in the direction of the drag, to resetting the strike plate. What you should not do is keep forcing the key until the bolt goes home. Every forced throw loads the tailpiece and the cam inside the cylinder with torque they were not sized for, and that is how an alignment problem becomes a snapped key or a cracked cam. If the bolt is starting to refuse to come back as well as go out, see deadbolt won’t retract because that page deals with the retraction side of the same misalignment.
If it fails only in one direction, then look at the cam and tailpiece
Some intermittent locks turn fine to lock and fight you to unlock, or the other way around.
If turning toward locked is smooth and turning toward unlocked sometimes stops partway, the cam or tailpiece at the back of the cylinder may be worn or loose. On a standard deadbolt, the cylinder drives a flat tailpiece that engages a slot in the bolt mechanism. When that tailpiece wears at its corners, or when the slot it rides in opens up, it can slip a few degrees before it catches. Sometimes it catches, sometimes it does not. The key turns freely for the first part of the rotation, then either finds the slot and throws the bolt or spins past without moving anything. A cylinder that occasionally turns all the way around with nothing happening is nearly always a tailpiece problem rather than a pin problem.
If the direction that fails is the direction that pulls the bolt back, and the bolt goes out fine, the spring inside the bolt housing may be weakening. That spring assists retraction, and as it fades, the key has to do more of the work. On a good day the key manages. On a bad day, with a little friction added from a swollen door, it does not.
If both directions fail with about the same frequency, the problem is more likely in the pin stack, and the next branch is where you want to be.
If it feels gritty or catches at a point, then the pins are the story
Set aside the door and think only about what the key feels like in the plug.
If the key goes in with a gritty, sandy feel and sometimes has to be pulled back a fraction and reinserted before it turns, the keyway has collected debris. In a coastal climate that debris is fine sand, salt crystals left behind by evaporated moisture, and pocket lint, all mixed with whatever lubricant has ever been sprayed in there. Oil based sprays are the worst offender because they hold the grit in suspension against the pins and then dry into a gummy film. The right first step is compressed air into the keyway, then a small amount of dry graphite or PTFE lock lubricant on the key blade, worked in and out several times. If the gritty feel improves and the intermittent failure goes away, you have found it. If the feel improves but the lock still fails now and then, debris was a symptom and wear is the cause.
If the key goes in smoothly but the turn catches at one consistent point and then either releases or does not, one pin stack is the problem. Pins wear into a slight mushroom shape at their tips, and springs lose tension. A worn pin sits a hair above or below the shear line when the key is seated, and whether the plug rotates depends on tiny variables like how firmly the key is pushed in. Jiggling the key up and down while turning often works on these locks, which is why people live with them for years. It is also a sign that the cylinder needs to be repinned before it seizes entirely.
If the key turns but feels loose or sloppy in the keyway, with more side to side play than it used to have, the plug itself and the keyway walls have worn. The key is no longer held in a single consistent position, so the pins are lifted to slightly different heights on every insertion. Some of those heights work. Some do not. A cylinder in this condition is a candidate for replacement rather than repinning, since new pins in a worn plug will still be positioned inconsistently.
If a copy stopped working but the original still does, then the copy is wrong
This branch is short but it saves a lot of wasted effort.
If a recently made duplicate works intermittently while the key it was copied from works every time, the duplicate was cut a few thousandths off in one or more positions. Duplicating machines drift out of calibration, and a copy traced from a worn original inherits the worn profile plus the machine’s own error. Stop using the copy. Every forced turn with a slightly wrong key drags the pins against the shear line and accelerates wear on a cylinder that was fine. Have the copy recut by code or from the original on a properly calibrated machine, and compare the two side by side under a light before trusting it.
If this is a car key, then the branches shift
Automotive locks and ignitions use spring loaded wafers instead of pin stacks, and they add an electronic layer that a house lock does not have.
If a car key sometimes will not turn in the ignition but always works in the door, the ignition wafers are worn or the steering column lock is binding. Turn the wheel slightly left and right while applying gentle pressure to the key. If that frees it, the column lock was loaded against the wheel and the ignition is probably fine. If the key still refuses, the wafers are hanging up and the cylinder needs service.
If a car key turns the ignition every time but the engine only starts sometimes, the mechanical key is not the problem at all. The transponder chip in the head of the key is being read inconsistently by the immobilizer, which is a different failure with a different fix, covered on the transponder key not recognized page. A common tell is that the failures cluster when the key ring is heavy or when another transponder key is dangling nearby, because a second chip can interfere with the read.
If none of the branches fit, then it may be the lock body rather than the cylinder
A small share of intermittent locks have nothing wrong with the key, the pins, or the door. The fault is in the mechanism behind the cylinder.
If a lever handle or knob sometimes turns without retracting the latch, and the key behaves the same way, a spindle, a retractor, or a spring cage inside the lock body has worn or broken. These parts do not fail all at once. A cracked retractor spring or a worn spindle hub engages most of the time and slips occasionally, and the pattern looks like an intermittent cylinder until someone takes the lock apart. On commercial lever locks that see hundreds of cycles a day, this is a more common cause than pin wear.
What not to do while you sort this out
Do not spray a general purpose penetrating oil into the cylinder. It will feel better for a week and then be worse than before, because it dissolves the old grease and dirt into a paste that migrates into the pin chambers. Do not force the key when it stops. A lock that fails one time in ten will fail three times in ten after a month of being forced, because the forcing rounds the pins faster. Do not file the key yourself in an attempt to make it fit better, since removing metal from a key is irreversible and the odds of removing it from the right spot without a gauge are poor. Do not loosen or remove the cylinder to look inside unless you know how the retention works, because some cylinders drop their pins and springs the moment the plug clears the housing, and the lock goes from intermittent to nonfunctional in one second.
When it becomes a locksmith job and what that visit looks like
Everything on this page up to filing a strike or replacing hinge screws is reasonable to try yourself. The moment the answer is inside the cylinder, the lock body, or the ignition, it becomes a locksmith job. A carpenter or handyman can rehang a badly sagging door, and if the whole frame has shifted, that trade may need to go first. But the lock itself is not a carpentry item.
A professional visit for an intermittent lock starts with the same branches described here, done faster and with better tools. The technician will test every key, compare the door open and door closed behavior, check the strike and hinges, and then pull the cylinder to inspect the plug and pins directly. Worn pins get replaced and the cylinder gets repinned to the existing key or a fresh one. A worn plug means a new cylinder, keyed to match the rest of the property so one key still does everything. A worn tailpiece, retractor, or spring gets swapped from stock when the hardware is a common line such as Schlage or Kwikset, and ordered when it is not. Ignition wafers get cleaned and replaced on the bench. Before leaving, the lock gets cycled dozens of times with every key, with the door open and closed, because an intermittent problem is only fixed when it stops being intermittent.
Preventing the next one
Keep the car key on a light ring, and keep house keys off the same ring as anything heavy. Have a fresh key cut from code every few years rather than copying copies. Once a year, blow the keyway out and use a dry lubricant, and never an oil. Watch the door in wet months and fix the first sign of drag at the strike before it loads the cylinder. Replace hinge screws with long ones the first time the door sags rather than the fifth.
If your lock has been giving you a jiggle-and-hope moment more than once a week, it is not going to improve on its own, and the branches above should have narrowed it down to one or two causes. Pro Locksmith serves homes and businesses throughout Broward County with 24-Hour Emergency service for exactly this kind of problem, and a call to (954) 406-8266 gets a technician who can confirm which branch you are on and fix it in one visit rather than waiting for the day the lock stops working entirely.