Five or six years ago, I was able to find a free OBDII diagnostic program by searching the web. But I did have to buy an adapter that went from the OBDII port on the car to a Serial connector on the adapter.
Then I had to go to an electronics store, and buy a male DB9 (computer serial port) connector, some multi conductor cable, and another DB9 connector to put on the computer end of the cable.
I also had to get a laptop with a serial connector on it, running a compatable, no longer supported windows operating system to run the program.
If I need to do this again, I will probably just buy something ready to use.