* Park Pawl Engagement: The most fundamental part is the physical mechanism inside the transmission. When you shift into park, a "park pawl" (a locking device) engages with a gear, physically preventing the car from rolling. This is a crucial safety feature.
* Park Position Sensor: A sensor, often a switch, detects the engagement of the park pawl. This switch is mechanically linked to the transmission's shift linkage. When the park pawl is engaged, the sensor closes, sending an electrical signal to the car's computer (Electronic Control Unit or ECU).
* Electronic Control Unit (ECU): The ECU receives the signal from the park position sensor. It uses this signal, along with input from other sensors (like the door sensors, seatbelt sensors, etc.), to determine if it's safe to allow unlocking.
* Immobilizer System: Modern cars have an immobilizer system that prevents the car from starting unless the correct key (or key fob) is present and recognized. The ECU also considers the immobilizer system status before allowing unlocking.
* Unlocking Mechanism: Once the ECU confirms that the car is in park and other safety criteria are met, it sends a signal to the central locking system, which unlocks the doors.
In short, it's a chain reaction: the physical act of shifting into park triggers a mechanical sensor, which sends an electronic signal to the car's computer, which then allows the unlocking system to operate. If any part of this chain is faulty (broken sensor, malfunctioning ECU, etc.), the car might not unlock.
