Until the mid-1990s, each car manufacturer had its own standards and protocols, making it difficult even for professional technicians to keep tabs on all the standards. That changed with the introduction of Onboard Diagnostics II (OBD-II), a standard adopted by automakers worldwide.
Despite the progress, professional scan tools can cost thousands of dollars, putting them out of range for most DIY mechanics. Up until a few years ago, even basic code and data readers cost hundreds of dollars. Simpler devices could read and clear codes, but they typically didn't provide access to the Parameter IDs (PIDs) that are often vital for diagnosing drivability problems and other issues.
The ELM327 programmed microcontroller is a small, relatively low-cost solution that bridges the gap. Devices that use this microcontroller, such as the Yongtek ELM327 Bluetooth scanner, don't compare with professional scan tools. Still, they put a lot of information in the hands of DIYers.