If your ELM 327 scanner won't "pair" with your phone, we're going to assume that your issue lies with the way that iOS devices access Bluetooth. If you purchased a generic ELM 327 device that uses Bluetooth as an interface method, the unfortunate reality is that it won't work with your unmodified iPhone. You may have more luck with a jailbroken device, although jailbreaking an iPhone just in the hope that it will work with your cheap ELM327 adapter probably isn't the best idea.

Better options are to spend money on an ELM327 scanner designed specifically for iPhones, buy a cheap Android phone or tablet, or just buy a separate OBD2 scan tool.

Most low-cost ELM 327 scan tools have a built-in Bluetooth chip, which is how they communicate wirelessly with a phone, tablet, or computer. The choice to rely on Bluetooth is largely due to the fact that both Bluetooth radios and the ELM 327 chip itself are cheap to produce, especially for manufacturers who use unlicensed, cloned versions of the ELM 327 chip instead of official ELM Electronics components.

If you can get a non-buggy unit with a working ELM 327 microchip, then the combination works quite well, as Bluetooth is more or less ubiquitous on mobile devices such as tablets, smartphones and even laptops these days. The main drawbacks of Bluetooth aren’t really an issue in these types of applications either, and the secure nature of the protocol means you don’t have to worry about someone secretly gaining access to information about your car.