What Is a Car Scanner? | Diagnostic Tool Explained

A car scanner is a diagnostic tool that plugs into your vehicle’s OBD-II port to read fault codes, monitor live sensor data, and help identify what’s wrong with your engine, emissions, and other key systems.

If your check-engine light just turned on, a car scanner is the fastest way to find out why. Also called an OBD-II scan tool, this device connects to your car’s onboard computer and translates diagnostic trouble codes (DTCs) into readable information—so you know whether it’s a loose gas cap or something that needs a mechanic. Before buying one, it helps to understand what these tools can and can’t do.

What a Car Scanner Actually Does

A car scanner retrieves diagnostic trouble codes from your vehicle’s computer and displays them as alphanumeric codes you can look up. Basic models read generic engine and emissions codes, which cover every gasoline car sold in the U.S. since 1996. Higher-end scanners also pull data from the transmission, ABS, airbag, and battery systems, and some can perform bi-directional tests—sending commands to components like fuel injectors to verify they’re working.

Most scanners also show live sensor data—engine RPM, coolant temperature, oxygen sensor readings—so you can spot problems while the engine runs. But here’s the limit: a car scanner diagnoses a problem, it doesn’t fix it. The tool tells you what’s wrong; the repair is still on you or your mechanic.

If you’re ready to buy, our tested roundup of the best car scanners breaks down the top models by price and feature set.

How To Use an OBD-II Scanner Correctly

Using one is straightforward, but the order matters. First, turn the ignition off before plugging the scanner into the OBD-II port—it’s a trapezoidal 16-pin connector usually located under the dash near the steering column. Once plugged in, turn the ignition to ON without starting the engine. The scanner will power up and establish communication with the vehicle’s computer.

Select Read Codes or DTC Scan on the device or app. The screen will display any codes the computer has stored. Write them down or save a screenshot before you clear anything. If the scanner asks for vehicle information, enter the VIN when prompted. Only clear codes after you’ve completed a repair or to verify a one-time issue—erasing them prematurely can destroy diagnostic clues.

OBD-II Scanner vs Code Reader: Knowing the Difference

These terms get used interchangeably, but they aren’t the same. A basic code reader simply pulls and clears engine-related fault codes—nothing more. A full OBD-II scanner (or scan tool) adds live data streams, readiness monitors, and often supports multiple vehicle systems beyond the engine.

The most common mistake people make is buying a basic code reader expecting dealer-level diagnostics. If you’re serious about DIY diagnostics, choose a scanner that matches your vehicle’s make and the systems you need to access. Some vehicles require scanners with specific protocols or adapters to read ABS or transmission codes.