Why does the car alarm go off when opening the door?

[McSara]

The reason the car alarm goes off when opening the door is: If the car is locked with a key, it can be unlocked with the key. Using the remote to lock the car is equivalent to activating the security system, and it can only be unlocked with the remote; otherwise, the alarm will sound. Additionally, using a spare key can also trigger this situation. More details are as follows: 1. For cars locked with a remote, when the door is opened with a mechanical key—whether it's the primary key or a spare key—the car's computer will interpret this as an intrusion because the car was locked remotely but opened mechanically, which it defaults to as a forced entry, triggering the alarm. If the car is both locked and unlocked with a mechanical key, the alarm will not sound. This is a security feature, and the alarm will stop once the ignition is turned on. 2. The remote key locks the doors and simultaneously activates the alarm, so using a spare key to open the door will cause the alarm to sound. Inserting the key quickly and turning on the ignition will stop the alarm. Since the spare key contains an engine decoding chip, the vehicle can be started and driven away. The spare key is intended for emergency use and should generally not be left inside the car.