What You Need to Know About OBD

Cars have been using computers to manage various systems since the 1970s, and the amount of systems controlled by digital computers has skyrocketed since then. New complications come along with new technology, meaning the advent of computer-controlled engines and powertrains necessitated a new diagnostic system for diagnosing malfunctions. That’s where OBD comes in.

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What is OBD?

On-Board Diagnostics (OBD) is a system that monitors a car’s computer system and reports diagnostic trouble codes. Devices like an OBD-II code reader can read diagnostic trouble codes (DTC) that the system stores when something goes wrong. Typically, at least one code is logged whenever the check engine light is illuminated. That’s how OBD helps mechanics and drivers rapidly diagnose issues with OBD-equipped vehicles.

The engine computer, or ECU, uses a microprocessor to read inputs from various engine sensors in real-time, and these readings are used to inform how electronically controlled automatic transmissions, traction control systems, and more react to real world driving situations to create the optimal driving experience. When a reading is outside of the expected values, the system logs a DTC which will be stored to be read through the OBD port.

1. Vehicles from 1981 to 1996 Use OBD-I

OBD-I refers to the first generation of on-board diagnostic systems, which featured proprietary connectors, hardware interfaces, and protocols. If you were a mechanic who wanted diagnostic information back then, you needed different equipment for each vehicle make, or a special OBD-I scan tool that supported multiple protocols and had adapters for each make. Regardless of which route you chose, you needed a lot of tools to work around the many OBD systems.

2. Vehicles from 1994 to Present Use OBD-II

OBD-II is a more standardized version of this system, making it easy to diagnose issues across different vehicles. Before The standardized OBD-II system eliminates the necessity for multiple adapters and scanner accessories. Now most OBD-II systems are nearly the same, but there are five different protocols with slight variations. The system was standardized with specifications from the Society of Automotive Engineers (SAE) to comply with California Air Resources Board (CARB) regulations.

These are the five OBD-II protocols:

  • SAE J1850 PWM: Pulse Width Modulation protocol is used in Ford vehicles
  • SAE J1850 VPW: Variable Pulse Width protocol is used in General Motors vehicles
  • ISO9141-2: All Chrysler and a variety of European or Asian vehicles use this protocol
  • ISO14230-4 (KWP2000): Keyword Protocol is used in a variety of European and Asian imports as well as Honda, Jeep, Land Rover, Subaru, Mazda, Nissan, and more
  • ISO 15765 CAN: Controller Area Network is used on all vehicles manufactured after 2008

What is an OBD Port and Where is It?

The OBD port is an electrical plug that OBD-II scanners, code readers, and other devices can be plugged into to access the OBD-II system. It is also the port that is typically used for state vehicle inspections. This port is typically located near or under the steering wheel column, although it is sometimes located near the center of the dash or center console, or under the glove box. If you’re having trouble finding it, check your owner’s manual.

Diagnostic Trouble Codes

When a code reader is connected to the port, it is capable of reading the diagnostic trouble codes that are stored in the system. DTCs typically start with a letter to indicate the system affected followed by five digits to specify the issue. There are codes for the powertrain, body, chassis, and network:

  • Powertrain codes begin with a P_____. Codes that begin P1____ are manufacturer specific, meaning they will indicate different things depending on the make
  • Body codes begin with a B_____
  • Chassis codes begin with a C_____
  • Network codes begin with a U_____

Some code readers will figure out what the code means for you, but if you just get the code, you may need to look up exactly what it means.

OBD for Performance and More

OBD was made for diagnostics, but it is also heavily used in tuner circles by those who are looking to get better performance out of their rides.

The system uses your vehicle’s sensors to determine whether something has gone wrong, but some sensors can even read real-time information from the sensors. Some companies even make dongles for the purpose of monitoring a vehicle’s system, whether for tuning or keeping tabs on a young driver’s speeds. There are also performance tuners, which connect to the OBD port and can remap the vehicle’s software to increase power output among other things.

Often, performance gains from OBD come at the expense of other qualities, like fuel economy and reliability.

What is an OBD-II Scanner?

OBD-II scanners are the tools that are used to read DTCs. The two most common types are:

  • Code Readers are basic devices that can read and clear codes from any OBD-II equipped vehicle. They are limited by the fact that while they can read codes, they typically provide no additional information, meaning they lack information on manufacturer-specific codes.
  • Scan Tools are more expensive and more versatile. They offer more troubleshooting information, can be used to diagnose manufacturer-specific codes, and can access live and recorded data.

There are many different scanners in these two types. While a scanner is definitely a useful tool for anyone working on cars built from 1996 on, not everyone needs the best scanner around. Some are capable of running an emissions test or other automated tests, some can read OBD-I and OBD-II, and some are dongles that wirelessly connect to a smart phone or computer. Consider what you need from a scanner and get one that meets your needs.

Troubleshoot With Fix Finder

If your check engine light is on and you need to figure out what’s wrong, come to your local AutoZone and ask about Fix Finder.

  • Fix Finder reads diagnostic trouble codes from your vehicle’s onboard diagnostic system, and pulls your vehicle information and mileage.
  • This data is analyzed to give you vehicle-specific fix solutions that have been verified from the records of over 5.5 million ASE-Certified technicians.
  • AutoZone gives you a FREE, easy-to-read MYZONE HEALTH REPORT, which includes relevant codes, and details the action that needs to be taken.

You can get the parts you need at your local AutoZone Store.

If the job is too big for you, seek out one of our Preferred Shops to help you do the job.

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.

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