If you have ever had a key cut at a hardware store and found that it unlocks the door but won't start the car, you have run into a transponder system. Here is what a transponder key is and how it talks to your vehicle. It also explains why a correctly cut copy still won't start the engine until it has been programmed. For service details, see our transponder key programming page.

Car won't recognize your key? Call (613) 707-4884 for help diagnosing it.

What Is a Transponder Key?

A transponder key looks like an ordinary car key, but the plastic head hides a small electronic chip. The chip has no battery. When the key is in or near the ignition, a coil around the ignition or push-button start sends out a weak radio signal. That signal powers the chip just long enough for it to answer with its identification code.

How the Key and the Car Work Together

The transponder is one half of the vehicle's immobilizer. Transport Canada describes an immobilizer as a system that, when armed, prevents the engine, fuel or ignition control unit from activating. It can only be disarmed by the right coded key, keypad or remote. The sequence is:

  1. You insert the key or press start.
  2. The car's antenna energizes the chip and reads its code.
  3. The immobilizer checks the code against the keys stored in its memory.
  4. If the code matches, the engine is allowed to run. If not, it may crank but will not start, or will start and stall.

Under Canada's motor vehicle safety standard CMVSS 114, immobilizers are required on passenger cars, multi-purpose passenger vehicles and trucks with a GVWR of 4,536 kg or less. Compliance has been mandatory since September 1, 2007.

Why a Cut Copy Isn't Enough

Cutting the blade only copies the mechanical shape. The new chip has no relationship with your car until it is registered in the immobilizer's memory, so it fails the code check. That is the difference between key duplication and replacement. Depending on the vehicle and the chip, this can mean cloning an existing key's code or adding a new key through the diagnostic port.

Signs Your Transponder Key Has a Problem

  • A security or key warning light stays on or flashes when you try to start.
  • The engine cranks but won't fire, or starts and dies within seconds.
  • A spare key starts the car but your main key doesn't. This points to a problem with that key's chip, especially if its head is cracked or damaged.
  • The car started with a new key once and then stopped recognizing it.

Note that remote buttons running on a weak battery are a separate circuit from the transponder. A dead fob battery usually doesn't stop the engine starting in a key-turn vehicle, so diagnose one problem at a time. Our article key fob not working? covers the remote side.

Why Programming Needs Professional Equipment

Adding a key means communicating with the vehicle's security system, usually through the diagnostic port. On some vehicles the procedure is short. On others it needs security access, a specific sequence, or erasing lost keys so they can no longer start the car. The wrong procedure can leave a vehicle with no working keys. A properly equipped automotive locksmith or a dealer should do this work, and only after confirming ownership.

Need a spare or a replacement transponder key? Call (613) 707-4884 with your make, model and year.

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