Arduino OBD library and software serial library
That library is hard-coded to use the single Hardware Serial device on the Arduino Uno or the second Hardware Serial1 device on other boards.

#ifndef OBDUART
#if defined(__AVR_ATmega328P__) || defined(__AVR_ATmega168P__)
#define OBDUART Serial
#else
#define OBDUART Serial1
#endif
#endif


You can edit the library's header file OBD.h to define your own serial port though:

#include &lt;SoftwareSerial.h&gt;
extern SoftwareSerial mySerialPort;
#define OBDUART mySerialPort


And then in your sketch create the mySerialPort SoftwareSerial object.