CAR KEYSWhat a transponder key is

A check that happens before the engine runs

Starting in the mid-1990s, manufacturers began putting a chip in the key and an electronic check in the car. It came in a few models at a time, and some base models went without it for many years after. The mechanical part of the key still works the way it always did: the cuts line up the pins or wafers and the cylinder turns. But on a car with this system, turning the cylinder isn't enough to start the engine.

The chip

Molded into the plastic head of the key is a small chip called a transponder. On most systems it has no battery. The car powers it wirelessly at the moment it's needed.

When you go to start the car, an antenna ring around the ignition wakes the chip up and reads it. The chip answers with a code, and the car's immobilizer checks it. On newer systems the car sends a different challenge each time and the chip has to come back with the right answer. If the code is recognized, the car is allowed to start. If it isn't, the car won't start normally. What exactly happens varies by manufacturer. Some cars start for a second or two and then die, some crank but never fire, some won't crank at all, and many flash a security light. The principle is the same: the car checks the key electronically before it will run.

Why a key that is only cut doesn't start the car

On a car with a chip system, a copy that's only been cut will often unlock the doors and even turn in the ignition, but the car won't start. The metal is right. The conversation isn't happening, because either there's no chip in that key or the car has never been told to trust the one that's there.

Cutting and programming are two different jobs

A working car key is really two jobs. It has to be cut so the mechanical part fits the lock, and it has to be programmed so the car recognizes the chip. Those are two different skills and usually two different pieces of equipment, and either one can be the reason a key doesn't work. Can you program your own?

The four main kinds of key you'll run into

• A plain mechanical key with no chip. Common on older vehicles.
• A transponder key, with the chip in the head and no buttons.
• A remote head key or flip key, with the chip and the remote buttons built into the same piece as the blade.
• A proximity or smart key, the kind you leave in your pocket. The car and the fob talk to each other without you touching the key, and most have a mechanical blade hidden inside for backup.

If you're buying a key

The chip type, the FCC ID and part number on a remote, and the blade profile all have to match the vehicle, and most keys sold online arrive uncut. Two remotes can share an FCC ID and still be different parts. Many used smart keys stay locked to the first car they were programmed to, so a used one may not program to yours. A key that looks identical on the outside can be the wrong one inside. Check the year, make and model against the key before you buy, and hold on to any key you still have working.

None of this is magic. It's just why "I'll get a spare made" turned into a bigger conversation than it used to be.