The ELM327 speaks serial UART on one end and CAN on the other, and takes simple commands via UART and translates them to the appropriate CAN/OBDII messages for the car.

There are a lot of protocol layers here, but the simplified picture is that the ELM327 is a sort of industry-standard chip, and it defines a protocol that, as others have said, is very frequently pirated or else re-implemented. Is there a way to map bluetooth to a usb port? I ended up paying more to buy my from reputable company and it worked on all the cars I have plugged it into. This is a classic example of buyer beware, and you only get what you are paying for. Even the reputable companies can not be 100% sure they getting the authentic ODBII chip in their modules if they are having them made in the China. The only way to be sure you have working module is to buy from a reputable company who will replace a unit if it does not work. Since most of these are Chinese knock offs and the people selling them has no connection to who makes them they have no idea which chips are actually used in the module. Some counterfeit chips work and others do not, so it hit and miss.

The reason some work and some do not even from the same seller or same company who makes them, is the fact they using counterfeit ODBII chips. From what I was able to find out it comes down to the ODBII chip used in these devices, they all claim to use the same chip (ELM327), however, some people say they work and other do not thus the reason for the mixed reviews.

and here is the long and short of these ODBII readers/interfaces. When I bought one of these I did lots of research due to all the complaints I read about.

Bmw inpa elm327 bluetooth