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

[LeJaxson]

The reason why the car alarm goes off when opening the door is: If you lock the car with the key, you can open it with the key when driving. Using the remote to lock is equivalent to activating the security system, and it can only be opened with the remote. Otherwise, the alarm will sound. The same situation occurs when using the spare key because for a car locked with the remote, when the door is opened with a mechanical key—whether it's the main key or the spare key—the car's computer will interpret it as an intrusion. Since the car was locked with the remote but opened mechanically, the system defaults to assuming a forced entry and triggers the alarm. If you lock and open the car with a mechanical key, the alarm won't sound. This is a safety feature. The alarm will be canceled after starting the engine. The remote key locks the door and simultaneously activates the alarm, so using the spare key to open the door will cause the alarm to sound.