Plug an automotive scan tool into the 16-pin OBD-II port, turn the ignition on without starting the engine, then read the codes.
For the full breakdown, see our best Truck Scan Tool guide.
The check-engine light is on, and the choice looks like a shop visit or a new tool. Learning how to use an automotive scan tool turns that into a ten-minute driveway job: connect, read, record, clear. Every 1996-or-newer car and light truck sold in the US uses the same OBD-II port and basic protocol, so the core routine is identical across brands.
How To Connect An Automotive Scan Tool
The safe order never changes: engine off, scanner plugged into the Data Link Connector, then ignition on without starting the engine. Every manual follows the same sequence.
- Turn the ignition off and let the car sit for a minute so every module goes quiet.
- Find the Data Link Connector — the 16-pin OBD-II port usually under the driver’s side of the dashboard near the hood release or fuse panel.
- Plug the scanner cable into the port. Most connectors are keyed, so they fit one way only.
- Turn the ignition on, but do not start the engine.
- Wait for the tool to detect the vehicle protocol and show its main menu.
The scanner powers up through the port itself, so no separate power source is needed for a basic read. One caution shows up in nearly every manual, from Foxwell’s to Harbor Freight’s: never attach or remove test equipment while the ignition is on or the engine is running. If the screen throws a LINK ERROR, turn the ignition off for about ten seconds, reseat the connector, and retry.
Reading, Recording, Then Clearing Codes
Once the main menu appears, pick the DTC or Codes option and the tool lists the stored diagnostic trouble codes. Write every code down first, because the clear function wipes everything.
CARFAX’s OBD-II code reader guide walks through the same sequence, and depending on the model, the menu also shows I/M monitor status — the readiness flags emissions stations check — plus deeper screens such as Vehicle Information, Live Data, Diagnostic Menu, or Rescan. Use the clear-codes function only after the repair is done. Clearing erases stored DTCs and the reset-related data that goes with them, so any code you skipped recording is gone. A successful clear turns off the check-engine light until the condition comes back.
Which Scanner Do You Actually Need?
The right tool depends on how deep you want to dig: a code reader pulls and clears engine codes, a live-data scanner adds real-time sensor readings, and a bidirectional tool sends commands to components. Menu depth is what separates them.
| Scanner Type | What It Does | Good For |
|---|---|---|
| Code Reader | Reads and clears engine DTCs | Check-engine-light checks |
| Live-Data Scanner | Shows sensor readings in real time | Finding why a code set |
| Bidirectional Scanner | Commands parts and modules | Component tests and module work |
Bluetooth scanners skip the cable and pair with a phone app, but the pairing and app setup come from the manual, and the app you install decides the feature set. Some tools ask for the vehicle make or VIN before they start — that’s normal, not a fault. Basic OBD-II scanners cover the generic engine and emissions data every compliant vehicle shares; manufacturer-specific menus for transmission, ABS, or airbags depend on the model and its software. If you work on a truck and want coverage beyond basic codes, our tested roundup of the best truck scan tools is worth reading before you buy.
Match the tool to the vehicle and the job, and the whole routine stays a ten-minute driveway task. Plug in, read, record, clear, and drive.
FAQs
Do I need to start the engine to read codes?
No. Turn the ignition to the on position with the engine off, and the scanner powers up and talks to the modules. Starting the engine during the initial connection goes against the sequence in most manuals and can interfere with the link or the readings.
Will clearing codes fix the problem?
Clearing codes only erases stored faults, so the check-engine light goes out until the condition shows up again. If the underlying issue is still there, the codes return, often within one drive cycle. Clear codes after the repair, not instead of doing one.
Can one scanner read every system on my car?
A basic OBD-II scanner reads the generic engine and emissions codes that every compliant vehicle must share. Manufacturer-specific systems such as ABS, airbags, and transmission diagnostics need a tool with added software for your brand, which varies by model. Check the manual or the compatibility list before you buy.
References & Sources
- CARFAX. “How to Use an OBD-II Code Reader for a Car.” Walks through connecting, reading, and clearing codes.
- The Home Depot. “How to Use an OBD2 Scanner.” Covers connection steps and scanner scope.
- Foxwell. “User Manuals.” Official manuals for NT-series scan tools, including connection cautions.