Lauren HollandBy Lauren Holland

Updated November 2026

Using an OBD2 scanner lets you find out why your check engine light is on without a trip to the shop. You plug the tool into a port under your dashboard, read the stored trouble codes, and look up what each one means. Clearing the codes is optional, and it’s best done only after you’ve fixed the underlying problem.

What You Need Before You Start

Most passenger cars and light trucks sold in the US since 1996 use the OBD2 standard, so a scanner will work on nearly any of them. Gather a few basics first so the process goes smoothly:

Shopping for one? See our guide: 3 Best OBD2 Scanners for Nissan Rogue Compared (2026).

  • An OBD2 scanner or code reader with a cable or Bluetooth connection
  • Your vehicle’s year, make and model
  • A notepad or phone to record codes exactly as they appear
  • Access to the ignition, with the engine off to start

How to Connect the Scanner and Read Codes

The diagnostic port is a 16-pin trapezoid connector, usually located within about two feet of the steering column, often above or beside the driver’s left knee. Some vehicles hide it behind a small cover, so check the door frame or lower dash panel if you don’t see it right away.

  1. Turn the ignition to the “on” position without starting the engine.
  2. Plug the scanner firmly into the port until it seats completely.
  3. Power on the scanner and select “Read Codes” or “DTC” from the menu.
  4. Wait for the tool to finish. Write down each code exactly, including the letter prefix (P, B, C or U) and all digits.

Understanding What the Codes Mean

Every code has five characters. The first letter identifies the system: P for powertrain, B for body, C for chassis and U for network communication. The first digit after the letter shows whether the code is a generic industry standard (0) or manufacturer-specific (1). Codes that start with P0 are standardized across brands, so a P0300 means random or multiple cylinder misfire on almost any car.

Look up the exact code plus your vehicle model, because the same code can point to slightly different causes depending on the manufacturer. A P0420, for example, often indicates a catalytic converter efficiency issue, but it can also be caused by an oxygen sensor or an exhaust leak.

Check Live Data Before Replacing Parts

Many scanners can show live sensor readings such as coolant temperature, short-term fuel trim and oxygen sensor voltage. Watching these values while the engine runs helps you confirm a suspected fault. A coolant temperature that never climbs above about 160°F, for instance, often points to a stuck-open thermostat.

Clearing Codes and Confirming the Repair

Once you’ve addressed the problem, use the scanner’s “Erase Codes” function to clear the stored trouble codes. Then start the engine and drive the car through a normal mix of city and highway speeds. The check engine light should stay off if the repair worked. If the codes return, the fault is still present.

Clearing codes without fixing the cause only hides the warning light. It also resets the car’s readiness monitors, which means the vehicle may fail an emissions test until it completes a full drive cycle.

Common Mistakes to Avoid

  • Clearing codes before writing them down, which erases the information you need to diagnose the fault.
  • Replacing parts based on a single code without checking live data or wiring first.
  • Ignoring a flashing check engine light, which usually signals an active misfire that can damage the catalytic converter.
  • Assuming a generic code reader shows manufacturer-specific codes. Basic tools often miss ABS, airbag and transmission codes.
  • Leaving the scanner plugged in while driving, which can drain the battery over time.

A basic handheld code reader is enough for most check engine light problems. A Bluetooth adapter paired with a phone app is useful if you want live data graphs and a larger screen. Keep a memory-safe jump starter in the trunk too, since a weak battery can trigger false codes.

  • Foxwell NT301 OBD2 Scanner: A straightforward handheld reader that displays codes and freeze frame data in a simple menu. View on Amazon
  • BlueDriver Bluetooth Professional OBD2 Scanner: Pairs with a smartphone app to show live data and enhanced manufacturer codes. View on Amazon

FAQ

Will an OBD2 scanner damage my car’s computer?

No. Reading codes and live data is a passive process that does not change how the engine runs. Stick to reputable scanners and avoid advanced functions unless you understand them.

Do I need to be connected to the internet to use one?

Basic code reading works offline on handheld scanners. Phone-based adapters may need the app installed, and some code definitions download from the internet.

Why does my check engine light go off by itself?

Some faults are intermittent, and the computer turns the light off after three consecutive drive cycles without the fault. The stored code may remain in memory until cleared, so check it before assuming the problem is gone. Consult a professional mechanic if the light keeps returning or you notice performance problems.

Conclusion

Using an OBD2 scanner is a practical way to find out what your car is reporting before you decide what to fix. Record every code, research it for your specific vehicle, and check live data before buying parts. Clear codes only after the repair is done and confirmed on the road.

Ready to choose one?

Lauren Holland

About Lauren Holland

Lauren Holland is part of our editorial team and reviews and edits our car care & garage supplies guides, checking product details against manufacturer specifications and owner feedback.