After moving the wrong black box out of the way, we were able to see the top of the bolt that secures the bracket for the Key Transponder ECU (red arrow). The photo doesn't really show how deep inside the dash the ECU is. Or how taut that wiring harness is above the bracket. Or how hard it is to press the release button on the electrical connector with my fat fingers. That being said, it's still a lot easier to deal with the above mentioned problems than it is to remove the entire dashboard to get better access.
When we got the call for a quote to replace the key transponder ECU, we checked the key registration procedure in the manual and it looked pretty easy. There was no flat rate time for replacing the Key Transponder ECU, but it looked like it would be under the lower portion of the dash on the driver's side and would be pretty easy to get to. As it turned out, we were wrong about that part. We quoted an hour labor. Whoops. After a more careful look at the manual, we realized that the ECU was in the upper portion of the dash. We pulled the cover off and found this (red arrow).
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.
