The door opened automatically. The verb in this sentence is-

An intransitive verb is defined as a verb that does not take a direct object.
That means there is no word in the sentence that tells who or what received the action of the verb.
আলোচ্য বাক্যে কাকে open করেছে বা কী open করেছে প্রশ্নের কোনো উত্তর পাওয়া যায়না।
অতএব, ‘open’ এখানে Intransitive verb.

Source: dictionary.com