How to Use an OBD-II Scanner

Reading an OBD-II code is the single most useful thing you can do when your check engine light turns on. The entire process takes less than five minutes and requires a tool that costs as little as $20 — or nothing at all if you use a free code-reading service at an auto parts store.

Types of OBD-II scanners

Where is the OBD-II port?

The OBD-II port (officially called the Data Link Connector, or DLC) is a standardized 16-pin trapezoidal connector found on every U.S. passenger car and light truck from 1996 onward.

It is almost always located under the dashboard on the driver's side, typically near the steering column or to the left of the steering wheel. On some vehicles it is behind a small cover panel. You don't need any tools to access it — it's designed to be plugged into without removing anything.

Step-by-step: reading OBD-II codes

1. 1
   Turn the ignition to 'On' (key on, engine off)

   You do not need the engine running to read codes. Just turn the key to the 'On' position — the dashboard lights up, but the engine is not started.
2. 2
   Plug the scanner into the OBD-II port

   The port is always under the dashboard on the driver's side. The plug should click in firmly — it will only fit in one orientation. For Bluetooth adapters, plug in and then pair with your phone app.
3. 3
   Power on the scanner and select 'Read Codes'

   Basic scanners will automatically begin reading. On more advanced tools, select your vehicle information (year, make, model) and then choose 'Read Codes' or 'Diagnostic Codes.'
4. 4
   Record all stored codes

   Write down every code that appears, including pending codes. A single check engine light can store multiple codes — diagnose them all, starting with any active codes.
5. 5
   Read the freeze frame data

   Freeze frame captures the operating conditions at the exact moment the code was set: engine RPM, vehicle speed, engine load, coolant temperature, fuel trim values. This is invaluable — it tells you whether the fault happened at idle, during acceleration, when cold, or at highway speed.
6. 6
   Look up what the codes mean

   Use Find This Code to look up each code. Every code page explains the likely causes, symptoms, and diagnostic steps.
7. 7
   Use live data to confirm the diagnosis (optional but recommended)

   Before replacing any parts, use the live data mode to monitor the affected sensor or system in real time. Confirm the sensor is reading incorrectly before spending money on a replacement.
8. 8
   Clear codes only after completing the repair

   Use the scanner's 'Clear Codes' or 'Erase Codes' function after you've fixed the problem. This resets the check engine light and the readiness monitors. Drive the vehicle through a normal cycle to confirm the code doesn't return.

Understanding fuel trim data

Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) are two of the most useful live data values for diagnosing lean and rich conditions.

When to use a professional scanner instead

A basic or mid-range scanner is sufficient for the majority of check engine light diagnosis. But there are times when you need a professional-grade tool or a shop:

• The code points to the transmission module (TCM) — most basic scanners only read powertrain codes
• You need to perform an active component test (commanding a solenoid to open/close)
• The code is a manufacturer-specific code (second digit = 1) that requires OEM software
• ABS, SRS, or body control module faults that basic scanners cannot access
• EVAP system testing requiring a smoke machine and active test capability