An OBD2 scanner plugs into the 16-pin connector under the dashboard, usually near the steering column. It communicates directly with the engine control unit. When a sensor reports a reading outside normal limits, the computer triggers a check engine light and stores a five-character code. The scanner reads these codes, giving you a starting point for repairs. Cheap scanners only display the code number, while professional systems show live data streams like coolant temperature, engine RPM, and oxygen sensor voltages.
Using a scanner saves hours of manual troubleshooting. However, reading a code does not automatically tell you which part to replace. A code for a lean oxygen sensor could mean a bad sensor, a vacuum leak, or a clogged fuel filter. You have to use the live data to see how the engine responds when you rev it or spray carb cleaner near the intake. Once you finish the repair, the scanner clears the code to turn off the warning light.