Home / After dark / When one key works and the other does not
Key diagnosis guide
When one key works and the other does not
Two keys, one lock, two different results. That puzzle usually has a clear answer once you know where to look. Work through the likely causes calmly, and avoid forcing the key that refuses to turn.
What the difference is telling you
When one key turns a lock and another does not, you have run a useful test without meaning to. The lock is the same, so the difference is in the keys. Either one is shaped correctly and the other is not, or one has something the other lacks, such as a chip that the car recognizes.
That narrows the problem a great deal. If the lock were failing, both keys would struggle, though perhaps in different degrees. When only one struggles, the troublesome key is the prime suspect, and the working key becomes a valuable guide. Keep it safe. It may be the most reliable reference anyone has for making a correct replacement.
This page covers the common causes, how to tell them apart without damaging anything, and what a locksmith does about each. It is written for a night situation, when you are tired, cold or in a hurry, because that is when people try to force the stubborn key.
Think about where you are, too. If you are at your own door in good light, you have time to test and compare. If you are in a dark lot or on a rural road, the priority shifts to safety: find a lit spot, keep your phone charged and let someone know. Our guide to waiting for help in a parking lot covers that case, and the plan for being locked out at night sets out the order of steps.
Start with the plainest check
Before looking for deep causes, confirm that you are using the right key in the right lock. On a ring of similar keys, it is easy to try the wrong one. Hold the two keys side by side under a good light. Are the blades the same length and shape? Do the grooves along the side match? Do the cuts look alike?
Check that the key is fully inserted. A key that stops a hair short will not lift the pins to the right height. Check that you are turning in the correct direction, and that the car is in park if you are at an ignition. Check that the steering wheel is not pressing on the lock. Our page on an ignition that will not turn, the steering lock explains that last one.
If all of that looks right and one key still fails, move to the causes below. Do not twist harder. A key that does not turn with gentle effort will not turn with force, and force is how keys break.
Check the temperature as well. Cold air shrinks metal and thickens the lubricant in a lock, so a marginal key that works on a warm afternoon may fail on a freezing night. If the stubborn key works indoors or after the lock has warmed, the key is borderline. That is one more sign it should be replaced, and not a sign the weather is to blame.
Cause one: a worn key
The most common reason is simple wear. A key used every day for years loses a little metal from its cuts. The edges round off, the valleys fill with grime and polish, and the key no longer lifts every pin to the right height. A spare kept in a drawer has none of that wear and works perfectly.
Look at the two keys together. If one has bright, smooth, rounded edges and the other has crisp, square cuts, you have found the cause. Our guide on worn key symptoms lists the other signs, such as wiggling, sticking and a gritty feel.
The cure is a new key. A locksmith can cut one by code or from the good key, and test it. Do not copy the worn key, since that passes the wear on. Do not use lubricant as a substitute. If the worn key is also your daily driver, replacing it protects the lock and avoids a break.
Do not try to fix a worn key with a file, sandpaper or a few taps with a hammer. The cuts are measured in tiny steps, and a change that you cannot see can make things worse. Also avoid the tempting idea that a key worn smooth can be made to work with extra wiggling. It may open the lock once, but it also drags the pins and speeds the wear on the lock.
Cause two: a poor copy
Not every key is cut well. A copy made from a copy, or cut on a machine that was slightly out of adjustment, can have cuts that are a fraction too shallow or too deep. A lock tolerates a small error, which is why the copy might work in the door but not the ignition, or on warm days but not cold ones.
Copies also differ in the blank. The metal blank must match the keyway of the lock. A blank from a similar but different family may fit the lock yet sit at a slightly wrong depth. The key looks right and feels right, but it does not turn.
The test is to compare the copy to the original. If the original works and the copy does not, the copy is wrong, and a better one should be cut. A locksmith may use the lock's code or a precise reading of the original, and can check the new key in the actual lock before leaving.
Cause three: a car key without the right chip program
On many vehicles, the blade is only half the key. The head carries a transponder chip that the car reads when the key is in the ignition or near the start button. If the chip is missing, wrong or not programmed to the car, the key may still turn the cylinder or open the door, but the engine will not start. In other cases, the blade turns the ignition lock only if the chip has been read.
That produces a puzzling pattern. A spare cut from a plain blank at a hardware counter might open the door and turn the ignition to the on position but refuse to start the car. The working key has a chip that the car knows. The other has no chip, or one that the car has never been taught.
Our guide on transponder keys explained for night emergencies explains this clearly, and the transponder programming page shows how a locksmith teaches the car a new key. Key types vary by model and year, so have the year, make, model and trim ready when you call.
Because vehicles differ, do not assume your key follows the same rules as a friend's. Some cars read the chip through the ignition, some through a small coil near the start button, and some through a radio link that the car checks when you press the button. The details depend on the model and year, which is why a locksmith asks for them before bringing blanks and programming tools.
Cause four: a fob that has lost its link or its battery
If your key includes buttons that lock and unlock the doors, the remote part can fail without the metal part failing. One fob may work while another does not because of a weak battery, a damaged circuit or a lost link to the car.
Try the simple fix first. Replace the battery in the weak fob, being careful to note the orientation and not to touch the circuit board with oily fingers. If the fob still does not work, it may need reprogramming. Some vehicles permit owners to do that with a procedure in the manual, while others require a locksmith's equipment.
In a push button vehicle, the fob also lets the car start. A dead fob can leave the car stubborn even when the car is otherwise fine. See key fob battery failure at night and push to start vehicles and lockouts for plain explanations.
If a fob stopped working after the car's battery was replaced or went flat, that may be the cause. Some vehicles lose their link to a remote when power is interrupted. The fob and the car need to be introduced again, which a locksmith with the right equipment can do. Mention any recent battery change when you call, since it points the diagnosis the right way.
Cause five: the lock is worn and one key fits its wear
A lock wears too. Over years, one particular key might have been used so often that the lock has shaped itself around it. Pins and wafers settle to the pattern of that key. A newer key, even one cut correctly, may not match the lock's worn state and may feel stiff or refuse to turn.
This is an easy trap to fall into. The old key seems healthy and the new key seems faulty, when the real problem is the lock. A locksmith can tell by testing both keys and by inspecting the cylinder. If the lock has worn to fit the old key only, the cure is to service or replace the cylinder, and cut keys to match.
For a house, rekeying can reset the lock to a fresh configuration. For a car, replacing a worn door or ignition cylinder may be the right step. Our pages on rekey locks and broken lock repair explain what those options involve.
Cause six: damage you cannot see
A key can be bent a little without looking bent. A fall, a pocket full of coins or a time when it was used to pry something can leave a slight twist along the blade. The key still goes in, and still looks fine, but it no longer lifts the pins evenly.
Lay the key on a flat surface and sight along the edge. Compare it with the good key. If there is a curve or a twist, that is the cause. Do not try to bend it back. Metal bent twice is weaker and likely to snap. Replace it.
Likewise, debris in the keyway can cause one key to fail while another passes. A bit of lint on one key's tip can hold the pins up. Wipe the key clean with a dry cloth and look into the keyway with a light. Do not poke at the keyway with a pin or a needle, because that can push debris farther in or scratch the pins.
A gentle way to test, without harm
You can narrow down the cause with a few careful steps.
Write down the answers, or say them aloud, because they are exactly what a locksmith asks. If the stubborn key fails everywhere, it is the key. If it fails in only one lock, that lock is suspect. If it fails only at the ignition, look to the chip or to the steering lock.
Stop the moment you feel gritty resistance or a catch. Pushing past that point is how a key snaps inside a lock. Read a key broken off in a door lock if you want to know what that looks like, and how to avoid it.
- Try both keys in the same lock, one after the other, using only light pressure.
- Try both keys in a different lock they should fit, such as the trunk or glove box.
- Look at both keys side by side under a bright light and compare cuts and edges.
- Check whether the stubborn key has a plastic head with a chip, and whether the working one does.
- Note when the failure happens: always, only when cold, or only in one lock.
What a locksmith does about it
A locksmith treats this as a diagnosis first. They confirm who you are and that you have the right to the vehicle or door, then test both keys in the lock. They inspect the blade, the cuts and the head. They look into the keyway and check the lock for wear or grit.
Based on what they find, they cut a new key, program a chip, replace a fob battery, service a lock or replace a cylinder. If a new key is needed, they cut it from the code or from the working key, and test it in every lock it serves. For a car, they also confirm that the engine starts and runs with the new key.
You can read more on vehicle key replacement, and on the vehicle locksmith service. Ask the locksmith to explain what they found in plain words. A good one does.
Do something about it before the second key fails too
If one key is bad and the other is your only good key, you are one failure from a lockout. The sensible move is to make a new, correctly cut key now, keep the good key as a reference, and store a spare away from the car. Our guide on why a spare key matters before you need it explains the habit, and adding a spare or replacing every key helps with the choice.
If you cannot be sure how many copies exist, or a key has been lost, think about a fuller reset. For a car, that can mean programming new keys and removing old ones from the car's memory. For a house, it can mean rekeying. Our guides on a lost key and vehicle security reprogramming questions and stolen keys go through those steps.
Redding Mobile Locksmith is a 24 hour mobile locksmith serving Shasta and Siskiyou counties, with a branded team and a service van carrying key cutting and programming equipment. Send your vehicle details and location through the request form.
Common questions
Why does my spare key work but my main key does not?
The most common reason is wear. The main key has been used for years, so its cuts are rounded and no longer lift the pins correctly, while the spare is still crisp. Compare the two under a light. If the main key looks smooth and shiny, a new key cut by code usually solves it.
Why does a copy of my key not work?
Copies can be cut slightly too deep or too shallow, or made from the wrong blank, especially a copy made from another copy. A lock tolerates only a small error. Take the original and the copy to a locksmith, who can cut a more accurate key from the lock code or a precise reading.
Could the stubborn key be missing its chip?
On many cars, yes. A plain metal copy may open the door and turn the ignition but not start the engine, because the car does not find the chip it expects. The key needs a chip programmed to the vehicle. Have the year, make, model and trim ready when you ask about it.
Should I force the key that will not turn?
No. Force can bend or snap the key and damage the lock. If gentle pressure does not turn it, stop. Try the other key, check you are using the right key, and look for wear. Forcing is how a simple wrong copy becomes a broken key inside the lock.
Can a worn lock be the cause instead?
Yes. A lock can wear to fit one particular key, which makes a newer or differently cut key feel stiff. A locksmith tests both keys and inspects the cylinder to tell. If the lock is worn, servicing or replacing the cylinder, or rekeying a house lock, restores smooth action.
What should I do if the working key is my only one?
Treat it carefully and have a spare made soon. Keep it off a heavy ring, avoid lending it, and use it as the reference for any copies. A new key made while the old one still works is far easier than replacing every key after you lose the last one at night.