Arduino AT command function

int8_t sendATcommand(char* ATcommand, char* expected_answer, unsigned int timeout){

    uint8_t x=0,  answer=0;
    char response[100];
    unsigned long previous;

    memset(response, '\0', 100); // Clean response buffer
    
    delay(100); // Delay to be sure no passed commands interfere
    
    while( Serial.available() > 0) Serial.read();    // Wait for clean input buffer
    
    Serial.println(ATcommand);    // Send the AT command 

    previous = millis();

    // this loop waits for the answer
    do{
        // if there are data in the UART input buffer, reads it and checks for the asnwer
        if(Serial.available() != 0){    
            response[x] = Serial.read();
            x++;
            // check if the desired answer is in the response of the module
            if (strstr(response, expected_answer) != NULL){
                answer = 1;
            }
        }
    // Waits for the answer with time out
    } while((answer == 0) && ((millis() - previous) < timeout));    

    return answer;
}

The function is only receiving an inverted question mark as a response? Any advise on how to fix this please?

I use this function in one of my projects since years and it works just fine for me and many others.
Your problem is somewhere else... Have you checked your serial speed ?

hello, when i see your code also i think this will work but, when i implement the code for example:
answer = sendATcommand("AT", "OK", 2000);

at serial monitor only AT and keep repeating , there's no "expected_answer" what am i doing wrong?