Automotive diagnostic equipment plugs into the cars computer system and reads the error codes it generates, giving the car owner or the mechanic information on the problem. It can be specialized handheld devices as well as mobile apps or laptop software. The cheaper models simply show the error codes, while the more expensive ones have more options for analysis.
Modern cars rely extensively on computers and have an internal communication network, called CAN-BUS. This network transmits information from sensors in the various components of the car to its central computer unit, including error codes when some part malfunctions. Automotive diagnostic equipment downloads this information from a port under the dashboard, either directly to the computer through a USB cable or to store on a memory drive for later use.
In the U.S. today, there is a uniform standard for On-Board Diagnostics, OTB-II. This means that the same diagnostic tools can be used on cars from any manufacturer. This has made them more accessible to the average car owner, whereas formerly they were almost exclusively used by professional mechanics.
However, diagnostic software still cannot necessarily identify the exact cause of the malfunction. It often cannot tell whether a component is broken or whether the electric connection to the part is at fault.