How to Use an OBD2 Scanner

Modern cars average between 30 and 50 modules that control electronic functions, and some luxury models have twice that number. They’re incredibly complex, and today’s technician is more of a computer tech than a mechanic, it seems. One of their main tools is an OBD2 scanner, and it’s a tool that the DIYer now needs to maintain and repair their vehicle, in most cases.

An OBD2 code reader is a handheld computer used to connect to your car’s onboard diagnostics system, which is where the acronym OBD comes from. OBDII, or OBD2, is the second generation of the in-car tech. As the modules monitor sensor readings and other data, values are displayed and recorded. If one of those values is outside of its normal range for longer than it should be, it triggers a fault code or diagnostic code. In many cases, the fault code triggers the warning light associated with that system such as the Check Engine light.

Whether you’re looking to clear a Check Engine light after a repair, diagnose why a warning light is on the dashboard, or monitor sensor values, you’ll need an OBD2 scanner. Here’s how you use it.

How to connect your OBD2 scanner

How to use an OBD2 Scanner

There’s only one place in your vehicle that a car code reader can connect: the OBDII port. For nearly all passenger vehicles, a female 16-pin connector shaped like a trapezoid is mounted under the driver’s side of the dashboard. In select instances, the connector could be hidden in a center console or under the passenger side of the dash.

Connecting the OBD reader is simple since the male end of the plug only fits one way. Match up the trapezoidal shape for the code reader and gently but firmly push them together. A slight wiggle might be necessary to fully seat the connectors together.

The order to connect an OBD2 scanner should be:

  • Ensure the ignition is off.
  • Connect the scan tool to the diagnostic port.
  • Cycle the ignition to the on position without the engine started.
  • The OBD2 port will power up your scan tool, in most cases.
  • Wait for the device to finish its boot sequence.

The scan tool will walk you through any additional information that’s required such as engine size, VIN number, or whether to turn the engine on or not.

Accessing the menu

Once your OBD2 scanner is booted up, you’ll have a menu to choose from. Depending on whether you have a basic code reader or an advanced device for monitoring systems and performing additional tests, the menu will look quite different. Commonly, you’ll have some or all the following options: 

  • Read Codes. This selection will present a diagnostic trouble code (DTC) number, and it may also provide a short description of the fault. If there are multiple codes stored or pending in the module, it will scroll through them all.
  • Erase Codes. To clear codes, you’ll navigate to this selection and choose it. Typically, it will confirm your selection before clearing the codes to ensure you didn’t accidentally press it. When you erase codes, it removes the failure data that’s stored in the computer.
  • Live Data. For a car code reader equipped with live data monitoring, you can scroll through sensor readings in real-time to see which values are in the right thresholds. It also can help you see when a sensor isn’t switching values at all. 
  • Freeze Frame. This is important information that reveals some common parameters at the time a DTC was set. 
  • Vehicle Info. This is simply data relating to your vehicle’s build.
  • I/M Readiness. Expanded to Inspection and Maintenance Readiness, this section can show areas that pertain to emissions and smog testing.

Your scanner will have several buttons on it, even if it’s a Bluetooth or Wi-Fi OBD2 scanner that’s controlled by your smartphone. Arrows will help you scroll to the selection you want to make, and another button will be pressed to make your menu choice. As well, a back button is often included to skip to a previous menu.

Learning what the codes mean

OBD-II scanner done reading trouble codes

When you select “Read Codes”, it shows a code that needs to be deciphered. There may be a brief description displayed on your screen, but not always. The format is always one letter followed by four numbers like this: P0301.

The letter indicates which system has experienced the fault. Options typically are: 

  • P, for powertrain systems like the engine, transmission, emissions, and fuel system.
  • B, for body control systems including airbag and other interior-related problems.
  • C, for chassis codes like suspension, power steering, and other exterior issues. 
  • U, for network-related issues.

The subsequent numbers are also quite descriptive. The first number is always either a 1 or 0. If it’s 0, it means it’s a generic code that applies to virtually all manufacturers. If it’s 1, it’s specific to the carmaker.

The second, third, and fourth numbers further identify the system and fault. The third digit shows the system:

  • 1 is for fuel and air metering
  • 2 is also fuel and air metering for injector circuits
  • 3 indicates an ignition system fault or misfire
  • 4 is related to auxiliary emission controls
  • 5 means there’s a vehicle speed control or idle system control fault
  • 7, 8, and 9 are all transmission-related codes
  • A, B, and C are all hybrid-specific codes

The third and fourth numbers further identify where the problem is, and with hundreds of potential combinations and descriptions. 

You can tell from this info that our example code, P0301, indicates a powertrain control module code, that the code is generic, and that it’s for the ignition system or a misfire. This code actually means that there’s a cylinder #1 misfire present.

How to save on auto repairs

With an OBD reader, you can save money in a few different ways. First, if you have a Check Engine light on or a performance-related issue, a quick scan can help reveal where the problem potentially lies. It doesn’t necessarily tell you what the problem is, but it’s the first step in a diagnosis.

Secondly, it can indicate whether your vehicle is prepared for a smog check. The I/M Readiness test often shows if there’s a fault that could prevent you from passing the test. 

As well, you can save money both on the diagnosis and the repair if you are able to track down the source of the fault without taking it to a shop. For example, a diagnostic scan that shows P0138 is saying that the bank 1 sensor 2 oxygen sensor has high voltage. Commonly, a new sensor is required to fix it. You can keep this repair a DIY fix when you can scan the codes on your own and clear them after the new sensor is installed. 

Get a new OBDII scanner at AutoZone and expand your DIY horizons. Or, explore our Loan-A-Tool program to borrow the tester at no charge. 

FAQ/People Also Ask

How do you use OBD II scanner?

Plug the scan tool connector into the connector under your dash, turn the ignition to the on position, and follow the instructions on the screen.

Does car have to be running to use OBD2 scanner?

No, in most instances, you can read trouble codes with the ignition in the ON position.

How do I read my OBD scanner?

Select “read codes” on your screen and mark down the diagnostic codes that are active and pending. The letter indicates the system involved and the numbers identify the specifics.

Can you plug in OBD scanner while car is running?

It’s possible to plug in an OBD scanner while the engine is on, but it isn’t advised. You may need to cycle the ignition off and back on to initiate the test.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

FREE Loan-A-Tool® program requires returnable deposit. Please note that the tool that you receive after placing an online order may be in a used but operable condition due to the nature of the Loan-A-Tool® program.

Related Posts