Car Will Not Start but the Key Works, What It Means
A car that locks and unlocks on the remote but will not start has a transponder problem rather than a remote problem, because those are two separate systems inside the same key. One runs on the battery in the fob. The other has no battery at all.
That is why the whole thing feels so contradictory. The key visibly works, so the key gets ruled out, and people start replacing batteries and starter motors instead. The half of the key that locks the doors and the half the engine depends on have almost nothing to do with each other.
One free test splits the whole fault in two, and it takes about a minute. What it points at, and what that costs on your particular make, is set out below.
Why the Remote Working Proves Nothing
There are two entirely separate systems inside one piece of plastic, and they answer to different parts of the car. Understanding which is which explains the whole fault.
Buttons, a circuit board and the small battery you can change yourself. It sends a radio signal to a receiver in the body of the car and its only job is locking, unlocking and the boot.
A tiny chip with no battery, sealed in the plastic. It is woken by a coil in the car at a range of a few centimetres and it answers with a code the engine will not run without.
They fail independently and for different reasons. A flat fob battery kills the remote and leaves the chip perfectly happy, and a cracked chip kills the engine while the buttons carry on locking the doors.
Every time somebody says the key must be fine because the doors locked, they are testing the wrong half. The half that matters cannot be tested by pressing anything.
This is why the most confident advice you will get from a friend or a passing mechanic is usually wrong. A key that locks the doors has proved that its battery is charged and its buttons work, and neither of those things is involved in starting the engine.
The Spare Key Test Splits It in Two
One test costs nothing, takes a minute and halves the possible causes. Do it before phoning anybody, because it changes who you should be phoning.
In the ignition, or against the start button on a keyless car. Give it a proper attempt rather than a token one, and note exactly what the engine does.
The car is fine. The first key has failed, and that is the cheaper half of this fault by a wide margin. Drive it, and get the first key sorted at your convenience.
Two independent keys do not fail on the same morning. Something on the vehicle has stopped reading them, and no amount of new keys will change that.
Hold the key against the ignition barrel or the start button and try again. If it fires that way, the reader is weak rather than the key being dead, and that is worth knowing before anybody quotes you.
A spare that starts the car turns this into a key job, which is the cheaper half. A spare that does not turns it into a vehicle job, which needs different equipment and a different quote. Knowing which before anybody travels is the difference between one visit and two.
If the Spare Works, the Key Has Failed
Four things go wrong inside a key, and only two of them mean buying a new one. The other two are repairs and they are considerably cheaper.
Usually a drop onto a hard floor or a spell in a washing machine. The chip is glass or ceramic and it cracks, and a cracked chip cannot answer the car however good the buttons feel.
The chip is perfectly good and the car has forgotten it. Common after a flat vehicle battery, a jump start or a previous programming attempt that did not finish cleanly.
A key that has only lost its pairing does not need replacing, which is the difference between £85 and a new key. That is established with equipment rather than assumed.
A key that turns badly in the barrel is mechanical wear and has nothing to do with the chip. It is worth fixing at the same visit rather than on a second one.
The pairing case is the one worth pushing on. A chip that is physically fine but no longer recognised is written back to the car rather than replaced, and it is common after a flat battery or a jump start. Transponder programming is what that involves.
If Neither Key Works, the Car Has Failed
Two independent keys do not stop working on the same morning by coincidence. Four things on the vehicle produce this, and the first two are far more common than people expect.
The coil around the barrel or start button that wakes the chip. As it fails it only reads at very close range, which is why holding the key against it suddenly works.
Enough to light the dash and lock the doors, not enough to hold voltage while cranking. The immobiliser drops out and refuses a perfectly good key.
A corrupted code in the module that stores which keys are allowed. This is the one that needs the chip read directly rather than through the diagnostic port.
On some platforms a failed steering lock motor or ignition switch reports as a security fault and blocks starting even though the key is being read correctly.
The battery one catches almost everybody, because the dashboard looks completely normal. Lights, radio and central locking draw very little current, and cranking pulls the voltage down far enough for the immobiliser to drop out and refuse a key that is perfectly good. That is checked under load before anything is opened up.
Where the fault really is in the module, it is read at the vehicle rather than recovered anywhere. Immobiliser diagnostics covers what that involves and which faults need the chip accessed directly.
What Has Failed on Your Make, and What It Costs
Every manufacturer gives this module a different name, and the name decides the equipment and the price. Six makes cover most of what gets attended across Leeds.
Prices are from, at your vehicle, with no VAT and no call out fee. Where an existing key only needs recoding the figure is lower than the vehicle side work shown here.
The Mercedes case is worth singling out, because it produces a fault that looks like a key problem and is not one. A failed steering lock motor on certain models blocks starting entirely while the key is being read perfectly, and no number of new keys will move it. Every make on the road is covered rather than just these six, and the full make list sets out the system each one uses.
What to Do Before Anybody Arrives
Four things, and all four shorten the visit or sharpen the price. None of them takes more than a couple of minutes.
Repeated attempts flatten the battery, and a flat battery on top of this turns one fault into two. Three or four tries is plenty to establish what is happening.
Strong spin and no fire, fires and dies after a second, or no crank at all. That single sentence narrows the job more than anything else you can tell somebody.
Proof of ownership is needed before key work starts. Locating it while you wait saves an hour, and on a company car it is rarely in the glovebox.
It identifies the module and the equipment needed, which fixes the price before anybody sets off rather than on arrival.
The car is not deteriorating while it sits, which is worth knowing if this has happened somewhere awkward. The engine is being prevented from running rather than failing to run, so nothing is wearing out and the car is completely normal again the moment the fault is cleared.
Questions Before You Call
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Doors Lock, Engine Will Not Start?
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