The RFID Immobilizer

The Toyota Prius uses an “immobilizer” system to prevent anyone from starting the car without a special key. The key contains a RFID (Radio Frequency Identification) chip in the head of the key. When the key is near the antenna loop around the ignition lock cylinder and the ignition switch is in the “on” position, the key will reflect an encrypted code to the key transponder ECU. The key transponder ECU checks to see if the key is registered as belonging to the car, then sends a message to the Hybrid Control ECU. The Hybrid Control ECU also checks to see if the key is registered. If it is, it sends a series of random numbers to the Key Transponder ECU. The Key Transponder ECU does some magical math, then sends the result to the Hybrid Control ECU. The Hybrid Control ECU checks the result and if it checks out, allows the car to start. Since the system is built into the Hybrid control ECU, it can not by by-passed or defeated. This prevents theft of the car by sneakily copying the key Mission Impossible soap impression style, or by simply hammering the ignition lock cylinder out Terminator style. The system can apparently be hacked (type RFID hack into Google) by those smarter and more determined than I am. However, towing the car would be a whole lot easier for those of not doing our doctoral work in RFID security.

So far everything sound great. Your car is safe. No one can steal it without buying a tow truck or diploma from MIT. If you could afford either, why would you want to steal someone’s car; right? But what happens if you lose one of your magical keys?

Losing one key is not a big deal, so long as you have at least one “master key”. A master key usually has a black head, and can unlock the glove box and the trunk, unlike the “sub key” AKA “valet key”. A master key can be used to register a new key with your Key Transponder and Hybrid Control ECUs. You can register up to 5 keys, and I’d recommend that you do, because…..

If you lose all of your keys, or all of your keys but the sub key, you will need to buy a new Key Transponder ECU to get your car to start again. As of 01/05, it’s cheaper to buy a key transponder ECU in a lock set including the ignition lock cylinder and two master keys than it is to buy just the key transponder ECU alone. Go figure. Since we were trying to do this as cheaply as possible, we planned to use only the ECU. That way we wouldn’t need to rekey the door and trunk lock cylinders. However, we would need to buy two new master keys and have them code cut to match the original locks.


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  • Anonymous

    You don’t have an equipment problem. You have a certification problem. Every Prius ever made has an immobilizer. But apparently nobody with adequate authority has placed the Prius on the proper list. By the way can I buy immobilizer on any autopartswarehouse?