What is a smart or proximity car key?

The short version

A smart or proximity key stays in your pocket. When you approach or touch the handle, the vehicle looks for the key nearby, confirms it is the right one, and allows the doors to unlock and the start button to work.

There is no blade to turn, though most smart keys hide a small emergency key and most vehicles have a backup way to start if the key's battery dies.

What usually goes wrong

• Battery — by far the most common cause of a key that has become unreliable.
• Water damage — smart keys and washing machines are a familiar combination.
• Physical damage — a dropped key with internal damage may work intermittently.
• Interference — in rare cases, nearby radio sources can affect reception.

Why compatibility is strict

Smart key systems are tied to platform and model year. Two cars wearing the same badge can use different frequencies or key profiles depending on when they were built and where they were sold.

That is why a key has to be matched to the vehicle before it is bought, not afterwards.

Spares for keyless cars

Smart keys are exactly the case where having a spare pays off, because a single failed key on a keyless car leaves you with no conventional fallback. Adding a second key while the first works is the simplest version of that job.