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What Is OBD2?


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Since 1996, the OBD2 System Has Enabled Drivers to Diagnose the Check Engine Light, Read Live OBD Data, and Much More.

OBD2 (or OBD-II) is short for On-Board Diagnostics version 2. It is an electronic system built into your car that constantly monitors engine performance and emissions. Through a variety of sensors, this system can detect problems and alert you to them by turning on the Check Engine Light. By using an OBD2 scanner that plugs into the OBD2 port usually located under your dashboard, you can get detailed information about your car problem from this system.

Originally intended for professional mechanics, anyone can now access this information and use it to repair your car more intelligently. Even if you don’t work on your own car, having this information when you take it to the shop will help you understand what’s wrong and avoid unnecessary repairs and upsells.


When Did OBD2 Start?

The first on-board diagnostics systems came along with the first electronic fuel injection systems. Volkswagen was the first manufacturer to introduce an on-board computer system with scanning capability in the 1969 Type 3 models. 

Datsun (now Nissan) followed in 1975 with the fuel-injected 280Z, and General Motors introduced their own system across its multitude of cars in 1980. All of these early OBD systems were only intended to connect to their own manufacturers’ scanning systems, which were only available at the factory or at dealers.

Starting in the late 1980s, the California Air Resources Board (CARB) started pushing the implementation of what would become today’s OBD2 system. First, they required all cars and trucks sold in California from 1988 on to have some sort of on-board diagnostic capability. At the time, CARB did not specify any standards for this system, though the Society of Automotive Engineers did suggest that the best way to accomplish CARB’s goal was to use a standard port and a standard set of electronic communication protocols across all manufacturers. 

In the mid-1990s, CARB saw the wisdom of SAE’s proposal, and incorporated it into a second on-board diagnostics requirement for all gas-powered cars and trucks as of 1996. This system is what we know today as OBD2. It has been the standard for automotive on-board diagnostics ever since.

Why Do We Need OBD2?

California was having serious smog problems in its major cities. CARB wanted to implement emissions testing for all cars and trucks, and saw on-board diagnostics as the best way to do it. California has more than twice as many vehicle registrations than any other state, and represents a huge market to car manufacturers. Any company refusing to comply with CARB’s requirements would lose a great deal of sales in that state. 

Additionally, other states began to jump on the bandwagon, implementing CARB’s standards for themselves as well. Currently, 14 states plus Washington, D.C. follow CARB’s emission standards for at least some of their counties and regions, if not all of them. 

Although the federal government never required the implementation of OBD2 in the United States, enough individual states did that manufacturers were forced to comply in order to sell cars in those states. Because OBD2 is fully integrated into a car, it made sense to include it in all cars sold in the US, regardless of whether they were sold in a CARB-compliant state or not. This also solves the problem of bringing a car originally purchased in a non-CARB state into one that is CARB-compliant.

OBD2 and emissions testing in general are responsible for the cleaner air in cities like Los Angeles, San Francisco, and others that used to have poor air quality. This is not only better for the environment, but it also makes it easier for us to breathe, particularly for people with respiratory issues such as asthma.

What Is the Difference Between OBD1 and OBD2?

OBD1 wasn’t called OBD1 when CARB mandated diagnostic systems in all cars and trucks sold in California. It was simply called OBD. When OBD2 replaced it in 1996, the original system was then called OBD1 to reduce confusion between the two systems. OBD1 is any system that meets CARB’s 1988 requirements for on-board diagnostics. Each manufacturer had its own way of meeting those requirements, and its own unique diagnostic port. 

Each brand of car had a different way to access diagnostic data. In General Motors cars, for example, you could stick a paper clip between two pins of the diagnostic port, turn the ignition on, and the check engine light would flash simple numeric codes that you could look up. For example, if it flashed one time, paused, flashed three more times, paused, then repeated the cycle, that was a code 13, which indicated a problem with an oxygen sensor. Honda had something similar, except you would uncover the ECU in the passenger footwell and watch codes flash on a light built into the ECU.

OBD2 is the standardized system that has been on all gas-powered vehicles since 1996 and all diesels since 2008. Every car uses the same OBD2 port, the same diagnostic codes, and the same communication protocols between the scanner and the car. You don’t have to know the special trick for your Pontiac or Mercury to get the trouble codes out of your car when the check engine light turns on. Any OBD2 scanner should work with any car, from a Ford Focus to a Lamborghini Murcielago.

Does My Car Have OBD2?

Generally speaking, all cars and trucks with gas engines from 1996 or newer have OBD2. Diesel cars and trucks 2008 or newer also have OBD2. You can enter your particular year, make, and model in our Compatibility Checker to see if your car has OBD2.

Where Is My OBD2 Port?

OBD2 requires a standard trapezoid-shaped port to be somewhere on or under the dashboard near your steering wheel. The exact location varies by make and model. Check out our OBD2 port location finder to find out where the port is in your car.

How Does OBD2 Work?

Modern cars are computer controlled. That computer is known as the ECU (Electronic Control Unit) or ECM (Engine Control Module). The ECU reads data from a wide variety of sensors throughout the car to determine the engine’s current operating conditions, and optimize the air and fuel mixture to make the engine run most effectively. 

By connecting an OBD2 scan tool or check engine light code reader into the OBD2 port on your car, you can “talk” to your car’s computer and find out what your check engine light means.

OBD2 Protocols

A communication protocol is a method that electronic devices use to communicate with each other. WiFi, Bluetooth, etc. all use protocols to connect to your OBD2 scanner, as well as send and receive email, play music on your wireless speaker, and encrypt your online banking information. OBD2 is the standard system for diagnostic devices to communicate with your car’s ECU. Within OBD2, there are five different communication protocols that may be used, depending on your car’s year, make, and model.

  • ISO 9141-2
  • KWP 2000-4
  • SAE J1850 PWM (also called Ford SCP)
  • SAE J1850 VPW (also called GM class 2)
  • ISO 15765-4 (also called CAN 11-bit and 29-bit)

Since 2008, all cars and trucks have been required to use the ISO 15765-4 protocol. Any OBD reader that supports this protocol will be able to connect to any 2008 or newer car. Many modern OBD2 scanners only support ISO 15765-4, not the other four protocols, as a cost saving measure. As a result, they may not be able to read codes on cars built before 2008, even though they’re all OBD2. If you have an older vehicle, it is important to make sure you use a scanner that supports all five OBD2 protocols.

OBD2 Diagnostic Trouble Codes

The ECU also detects when problems occur, such as a misfire. It saves to its memory the exact nature of the problem as an OBD2 code, also referred to as a Diagnostic Trouble Code (DTC). In the case of a random misfire on multiple cylinders, the ECU will record the code P0300. It will also turn on the check engine light on your dashboard to alert you that there is a problem. When you plug in an OBD2 scanner, you will be able to read the code to tell you exactly what the problem is.

The OBD2 codes themselves have a specific format to further identify the type of problem they correspond to. Not all codes apply to all vehicles. A front-wheel drive car should not register any problems with the all-wheel drive system, for example. But because OBD2 is a universal system, all cars use the same set of trouble codes.

  • B: Body. Codes in this category also include any problems with safety systems like airbags and seat belts. Some hybrid cars also use codes in this category for problems with the hybrid battery system.
  • C: Chassis. These include systems not related to the powertrain but that are still important to the safe operation of the vehicle.
    • C00: Anti-lock brakes.
    • C01: Brake hydraulic systems.
    • C02: Wheel speed sensors and traction control.
    • C03: Four-wheel drive or all-wheel drive.
    • C04 and C05: Steering.
    • C06 and C08: Suspension and self-leveling.
    • C07: Tire pressure.
  • P: Powertrain. These codes relate directly to the engine, transmission, and emission system performance.
    • P01: Fuel and air metering.
    • P02: Fuel injection system.
    • P03: Ignition system.
    • P04: Auxiliary emissions control, such as oxygen sensor heaters.
    • P05: Vehicle speed control and idle control.
    • P06: Computer output circuits.
    • P07 and P08: Transmission.
  • U: Network. Codes in this category identify problems within the on-board diagnostic system itself, specifically with data transmission between different sensors and systems.
    • U00: Communication bus.
    • U01 and U02: Lost communication with sensor.
    • U03: Software incompatibility.
    • U04: Invalid data received.

The final two digits in each of these codes indicates the exact nature of the problem. For example, while P0300 indicates a random misfire in multiple cylinders, P0301 is a problem specifically in cylinder 1. P0300 could be caused by a more general issue like a fuel problem or bad ignition coil, but with a P0301 code, you can begin your diagnosis at cylinder 1 with issues like a bad spark plug or wire. This is just one of countless examples of how knowing the trouble code can help you diagnose a problem more quickly and easily if you know what to look for.

How to Read OBD2 Live Data

Reading trouble codes are good, but that’s not all OBD2 can do. You might have seen a mechanic look around under the hood while your car is running to try and track down a problem while it’s actually occurring. 

OBD2 lets you do a high-tech version of this, except rather than sticking your head in the engine bay, you can hook up an car scanner to read live OBD2 data.This, in conjunction with reading a trouble code, is the best way to properly diagnose what’s wrong with your car.

One common example of how this can help is with the common P0420 code. This code indicates that your exhaust system’s catalytic converter isn’t working properly. However, it’s very common for this code to appear as a result of a failed oxygen sensor rather than the catalytic converter itself. A good way to determine if this is the case is to view the live data for the oxygen sensor and see how it behaves.

Watch the voltage of the oxygen sensor. It should fluctuate between a reading below half a volt and above half a volt every two to five seconds. If the voltage changes more slowly or not at all, there’s a good chance that your sensor is bad. (Believe it or not, this is actually a good thing. An oxygen sensor is much cheaper and easier to replace than a catalytic converter. Failed oxygen sensors are also responsible for almost half of all emissions system test failures.)

If the sensor voltage fluctuates as it should, your problem lies elsewhere. It’s always best to check the most obvious issues first, though. It’s always worth reading live data through your OBD2 scanner to check the oxygen sensor first when dealing with any problems it may be connected to.

What Is an OBD2 Scanner?

The device you plug into your car’s OBD2 port to read its check engine codes and live data is generally known as a scanner. These take many forms, ranging from a tiny box barely bigger than the OBD2 port itself, to a large device connected to the port by a wire. 

In locations that test your car’s emissions system, the computer that plugs into your car, communicates with the state’s auto registration database, and prints your new inspection sticker is also a type of OBD2 scanner.

What Are the Best OBD2 Scanners?

Unless you’re a professional mechanic, you should get most of the diagnostic OBD data you need from a quality OBD2 scanner under $100

For the average driver and even experienced DIYers, the FIXD OBD2 scanner is both powerful and extremely easy to use. Just plug in the scanner, install the free app, follow the instructions, and it just works. The data display inside the free FIXD app is much more user-friendly than other scan tools. It features a mix of text and visual data, including charts and graphs, that make it easy to understand what’s going on inside your vehicle.

FIXD not only reads most engine codes but also translates them into plain English so that anyone can understand what they mean. If you have a trouble code, FIXD gives you a list of possible causes and links to in-depth OBD2 code descriptions and DIY videos to help you fix simple issues yourself. 

FIXD has scanned more than 18 million vehicles for check engine lights, but it’s far more than just a check engine code reader. It also has Automated Maintenance Alerts so you never forget an oil or filter change. Plus, Multi-Vehicle Health Monitoring lets you connect multiple FIXD Sensors to every car you own and monitor their health in real-time from your smartphone.

Different people have different needs, however, and FIXD may not be the right choice for everyone. If you’re interested in customizing the comfort and convenience features of your car, Carly might be the right choice for you. To increase your horsepower and performance, a device like the Cobb Accessport that plugs into your OBD2 port is well worth considering. If you’re on a tight budget, or you simply prefer to use independent third-party software, the ELM327 and whatever app you choose might do the trick for you.


The OBD2 scanners and functionality available today are capable of far more than the limited professional use that OBD2 was designed for in the first place. Anyone with a smartphone can now access much of the same data as a shop with a much more affordable scanner. Tools like the FIXD OBD2 scanner not only read diagnostic data but also translate the technical codes and jargon into plain English that anyone can understand, as well as display data in real time. Customization options ranging from turning on the interior lights to increasing engine horsepower are possible through what used to be just a diagnostic port. Sometimes technology really does make our lives easier.

Popular Engine Codes P0420 P0172 P00171 P0128 P0300 P0456 P0430 P0442 P0455 P0113

Justin Hughes

Recovering autocross and track day enthusiast. Once turned a VW Jetta into a pickup truck. Lives in a van down by the river. Dream car: 2001 Subaru WRC rally car.

We’re here to help you simplify car care and save, so this post may contain affiliate links to help you do just that. If you click on a link and take action, we may earn a commission. However, the analysis and opinions expressed are our own.

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About the Author

Justin Hughes

Justin Hughes

Recovering autocross and track day enthusiast. Once turned a VW Jetta into a pickup truck. Lives in a van down by the river. Dream car: 2001 Subaru WRC rally car.

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