“The Carista-branded adapter is the only one we tested, then, that offers both iPhone compatibility and the extended Carista “mod” functions. The other adapters work very well for OBD-II diagnostic work in apps like OBD Fusion.

Cheap adapters are almost all based on the ELM327 design, but Elm charges $15 just for the chip in bulk, so it shouldn’t be surprising if a $9 buy-it-now-bargain doesn’t use a genuine chip.

Specialized designs from companies like Carista, PLX Devices and OBDLinkare designing circuits around new, 100% legitimate versions of Elm’s chip, which means lower power draw and a larger buffer for data transmission, along with a few new commands. These adapters are more likely to use the new Bluetooth Low Energy and Bluetooth 4.0 standards, which also brings iPhone compatibility.

For example, take the Volkswagen “hatch pop mod.” With parts from an enthusiast parts supplier like ECS tuning and the Carista or OBDeleven app, you can add a hatch open remote to a VW Golf that didn’t come with that feature from the factory.”

In fact the review team looked at how compatible other software is and made it the Carista Compatible category — “Compatibility is the trickiest part of picking a tool for extended diagnostics.”