What is a Transponder Key?
If you get a new key made for your car, you may be shocked to find that the car won't actually start even after you put the key in and turn the ignition. In this case, the problem is usually that your new key hasn't been programmed correctly. While an improperly programmed or even unprogrammed key may let you into the vehicle after you've locked keys in car, it won't start it.
The problem in this case is that your car requires a transponder key. These keys respond to a radio signal sent by the car itself when they are near or in the ignition. If the response is wrong or nonexistent, the car realizes that it's not an authorized version. In order to thwart thieves that copy keys, the car won't start.
This does more than make it harder to get a new key that will actually work. It also increases the cost of replacements since whoever makes them has to have programmable key blanks or fobs, know how to do the programming, and take the time to accomplish it. If you go to the dealer for the service, it can cost you several hundred dollars, with the exact price varying both by car model and dealership.
While there is no way to avoid this problem entirely if you've lost your key instead of simply having locked keys in car, you can control the costs by having a mobile locksmith come and make your new ignition key. Locksmiths generally charge less than dealers, and the mobile ones even save you a trip. It's a good idea to have two transponder key copies made if you need any, so that you will have a spare.