OBD2 scanner vs code reader: what the difference means for you

Code reader: read and clear engine codes

A code reader is the simplest tool. It plugs into the OBD2 port under the dash, reads the diagnostic trouble codes stored by the engine computer, shows a short definition and lets you clear them, which turns off the check engine light until the fault returns. Most also show readiness monitors, the self tests that must be complete for an emissions inspection.

That covers the generic engine and emissions side of OBD2 that every 1996 and newer US car must support. A code reader tells you, for example, that you have P0420 or P0171. It does not tell you why. To find out whether P0171 is a vacuum leak or a weak fuel pump, you need to see sensor data, and most basic code readers do not show much of it. See how to read OBD2 codes for what each part of a code means.

OBD2 scanner: live data, freeze frame and more systems

An OBD2 scanner, or scan tool, does everything a code reader does and adds the information you need to diagnose. The key extra is live data: real time readings like coolant temperature, engine speed, fuel trims, oxygen sensor voltage and airflow. Scanners also show freeze frame, the snapshot of conditions the computer saved when a code set, and many can graph and record data to catch intermittent problems.

Better scanners read more systems than the engine. Four system tools add transmission, ABS and airbag (SRS). All system or full system tools reach body, climate, steering, TPMS and other modules using each maker's enhanced codes, and often include service resets like oil life, electronic parking brake and steering angle. Learn to use the data in freeze frame data explained and fuel trims explained.

Bidirectional scanner: active tests

A bidirectional scanner can send commands, not just listen. It can turn on a fuel pump, cycle the cooling fan, open an EVAP purge valve, pulse an ABS pump for brake bleeding or move a throttle body. These active tests let you check whether a part and its wiring respond before you replace anything. If the fan runs when commanded, the fan and its relay are likely fine and the problem is in the sensor or logic that should turn it on.

Bidirectional tools cost more, but they save time and parts on real repairs. Read our explainer, what is a bidirectional scan tool, for what these tests can and cannot prove.

See the best bidirectional scan tools →

Side by side comparison

Sellers use these names loosely. Some tools sold as code readers show live data, and some called scanners only cover the engine. Check the function list rather than the product name.

Where phone apps and dongles fit

Bluetooth dongles that pair with a phone app, including subscription based products such as FIXD, sit somewhere between a code reader and a scanner. The basic function is reading and clearing engine codes, and many apps show some live data. What you get beyond that depends on the app and often on a paid plan, so check whether ABS, airbag or other systems are included and what the ongoing cost is.

If you like the phone or tablet format but want full system access, there are wireless scan tools with their own apps or tablets. Our wireless and Bluetooth scan tool guide covers those. Also read scan tool updates and subscriptions before you commit to any tool with a yearly fee.

Which one should you buy?

Our picks for each level are in the best OBD2 scanner for DIY mechanics guide and the ABS and SRS scanner guide. Examples from those guides include the Foxwell NT624 Elite, an all system scanner with 8 listed resets, and bidirectional tools such as the Innova 5610 and Autel MK808S. Once you have a tool, our guide on how to use an OBD2 scanner covers the basics.

• Code reader: you want to know why the check engine light is on, check readiness before an inspection, and leave repairs to a shop.
• All system scanner: you do your own repairs and want live data, freeze frame, ABS and airbag codes, and common service resets. This is the sweet spot for most DIY mechanics.
• Bidirectional scanner: you diagnose problems beyond the engine, bleed ABS brakes, or want to test parts before buying them.

Frequently asked questions

Is a code reader enough to fix my car? It is enough to learn the code. To find the cause, you usually need live data and freeze frame, which means a scanner.

Can a code reader turn off the check engine light? Yes. Clearing the code turns the light off, but it comes back if the fault is still there. Clearing also resets readiness monitors, so do not clear codes right before an emissions test. See how to clear the check engine light.

Do I need a bidirectional scanner for DIY work? Not always. It is most useful for ABS brake bleeding, EVAP tests and checking pumps, fans and solenoids. If you only do maintenance and engine codes, an all system scanner is usually enough.

Will a code reader read ABS or airbag lights? Usually not. Basic readers cover the engine and emissions system. ABS and SRS codes need a scanner that lists those systems.