MH-Z14 ESP8266 Arduino example

mh-z14.ino

#include <ESP8266WiFi.h>
#include <WiFiClient.h>
#include <ESP8266WebServer.h>
#include <ESP8266mDNS.h>
#include <time.h>

const char* ssid = "co2_sensor";
const char* password = "1qazxsw2";

ESP8266WebServer server(80);

const int led = BUILTIN_LED;
int getCO2();

int pwm_co2 = 0;

void pwm_in_interrupt() {
  static int t_last = 0;
  int t = micros();
  if (digitalRead(D5) == 0) {
    pwm_co2 = (t - t_last - 2) * 4 / 1000;
  }
  t_last = t;
}

void setup(void) {
  WiFi.mode(WIFI_AP);
  pinMode(led, OUTPUT);
  digitalWrite(led, 0);
  Serial.begin(9600);
  Serial.swap();
  WiFi.softAP(ssid, password);
  attachInterrupt(D5, pwm_in_interrupt, CHANGE);

//  // Wait for connection
//  while (WiFi.status() != WL_CONNECTED) {
//    digitalWrite(led, 1);
//    delay(400);
//    digitalWrite(led, 0);
//    delay(100);
//  }

//  if (MDNS.begin("esp8266-mh-z14")) {
//    MDNS.addService("http", "tcp", 80);
//  }

//  configTime(3600 * 3, 0, "pool.ntp.org");

  server.on("/", []() {
    digitalWrite(led, 0);
    int co2 = getCO2();
//    time_t ts;
//    time(&ts);
    int ts = millis();
    String response = String("{\"co2\":") + co2 + ", \"pwm_co2\":" + pwm_co2 + ", \"time\":" + ts + "}\n";
    server.send(200, "text/json", response);
    digitalWrite(led, 1);
  });
  server.begin();
  delay(5000);
  digitalWrite(led, 1);
}

void loop(void) {
  server.handleClient();
}

const uint8_t cmd[9] = {0xFF,0x01,0x86,0x00,0x00,0x00,0x00,0x00,0x79};
int getCO2() {
  uint8_t response[9];
  Serial.write(cmd, 9);
  if (Serial.readBytes(response, 9) == 9) {
    int responseHigh = (int) response[2];
    int responseLow = (int) response[3];
    int ppm = (256 * responseHigh) + responseLow;
    return ppm;
  }
  return -1;
}

I'm having problems reading the UART register. It looked like the values were correct once but then switched to this:
Address 0 FF
Address 1 86
Address 2 7
Address 3 D0
Address 4 43
Address 5 0
Address 6 0
Address 7 0
Address 8 60
I sent the device this request
byte addArray[] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};