How to Read Check Engine Codes With an OBD2 Scanner

Difficulty: Beginner Time: 5–10 minutes

Every car sold in the US since 1996 has an OBD2 port that stores a trouble code when the check engine light comes on. Reading that code is the first step to figuring out what is wrong, and it takes about 5 minutes.

Basic code readers are inexpensive, and many auto parts stores will read the code for free. The code tells you which system has a problem, not always the exact part, so it is a starting point rather than a final answer.

What you'll need

Step by step

1. 1

   Find the OBD2 port

   Look under the dashboard on the driver’s side, usually near your knees or above the pedals. It is a trapezoid-shaped plug with 16 pins. A few cars hide it behind a small cover.

2. 2

   Turn the car off and plug in the scanner

   With the ignition off, push the scanner’s plug firmly into the port. It only fits one way.

3. 3

   Turn the key to ON

   Turn the key to ON (or press the start button without your foot on the brake) so the dashboard lights come on. Most scanners do not need the engine running.

4. 4

   Read the codes

   Follow the scanner’s menu to "Read codes" or "DTCs." Codes look like one letter and four characters, for example P0420.

5. 5

   Write every code down

   Write down every code, including any marked "pending." If your scanner shows freeze frame data (conditions when the code set), note that too.

6. 6

   Look up what the code means

   Look up each code in GarageGuide’s code guides for what it means, how serious it is, and what to check first.

7. 7

   Don’t clear the code yet

   Clearing the code turns off the light but does not fix anything, and it erases information a mechanic could use. Clear it only after a repair, to confirm the problem is gone.

8. 8

   Unplug the scanner

   Turn the ignition off and unplug the scanner.

Tips

• If the check engine light is flashing, that means an active misfire. Do not keep driving hard; read the code and get it fixed soon.
• Inexpensive Bluetooth scanners work with a phone app and can show live data like engine temperature.
• If the code appears again after you clear it, the problem is still there.

Parts and tools that may help

Enter your year, make and model on the store's site to see parts that fit your car.

OBD2 code readerPick up today · Advance Auto Parts

Gas capPick up or ship · Advance Auto Parts

Spark plugsPick up or ship · Advance Auto Parts

Ignition coilPick up or ship · Advance Auto Parts

Oxygen sensorPick up or ship · Advance Auto Parts

MAF sensor cleanerPick up or ship · Advance Auto Parts

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Not sure that's the problem?

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Frequently asked questions

Where is the OBD2 port on my car?

Almost always under the dashboard on the driver’s side, near the steering column or above the pedals. Your owner’s manual shows the exact spot.

Will an auto parts store read my codes for free?

Many will. They plug in a scanner in the parking lot and tell you the code. Just remember the code shows the system with the problem, not always the part to buy.

Can I just clear the check engine light?

You can, but the problem is still there, and the light will come back. Clearing it also resets your emissions readiness, so you may fail an inspection right after.

Does the engine need to be running to read codes?

Usually not. Turning the key to ON is enough for most scanners. Check your scanner’s instructions.

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