What is OBD and How Does it Work?

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Olga Weis Last updated Nov 23, 2022

What is OBD and How Does it Work?

Have you ever wondered how your vehicle can tell the amount of fuel left? Or where the "check engine" light (that wouldn't just go away) comes from? These indicators are the result of a complex diagnostic system called OBD, which is used to monitor a vehicle's health.

On-Board Diagnostics is a self-diagnostic and reporting system that helps repair technicians access the status of various subsystems within an automobile. Aside from diagnostics, an OBD system can regulate a vehicle’s performance and perform minor repairs itself.

OBDs collect data about the vehicle's state, process it, and transmit the information to internal and external observers. This information is encoded in a standard format for universal analysis and interpretation. As such, mechanics can accurately assess a vehicle's repair needs.

What is an OBD Port?

An OBD port is a 16-pin port, usually found under the driver’s side dashboard that accepts various computers and diagnostics devices. OBD port pinout may vary from manufacturer to manufacturer. Technicians connect an OBD-compatible adapter to this port to monitor vehicle activity and receive errors in a human-readable format.

What Type of OBD Is My Car?

There are two standard types of OBD; OBD1 and OBD2. If you want to know what type of OBD your vehicle uses, you can:

Check the year your car was manufactured. Prior to 1996, most vehicles used OBD1. However, all vehicles manufactured from 1996 are required to use OBD2. This method may be unreliable as some vehicles were slow to make the transition.
Check your vehicle’s OBD port. Standard OBD2 ports have connectors with 16 pins and look similar to other OBD interfaces. If your vehicle uses anything different from a 16-pin connector, it most likely uses an OBD1 system.
The most reliable method of checking your vehicle’s OBD type is to read the owner’s manual. If you’re missing your owner’s manual, you can check beneath the hood for a label stating if your vehicle uses the OBD1 or OBD2 system.

OBD-I

OBD-I is a family of first-generation on-board diagnostics systems developed in the 1980s to regulate vehicle emission control systems. However, most OBD-Is were unsuccessful in performing this task due to a lack of standardization. Very few, if any, of these early systems shared components with other OBDs. Rather, most manufacturers used proprietary hardware and protocols.

As a result, repair technicians had to buy a separate tool for every vehicle model. They also used an array of adapter cables to interface with OBD-I scan tools that supported multiple protocols.

OBD-II

OBD-II significantly improves OBD-I systems in terms of standardization and functionality. All vehicles manufactured from 1996 onwards must be OBD-II compliant and follow a set of standards published by the Society of Automotive Engineers (SAE) and the International Standardization Organization (ISO).

These standards describe the nature of information exchange between ECUs and diagnostic scan tools. They also stipulate standard diagnostic connectors and communication protocols. OBD-II also introduced standard diagnostic trouble codes and data logging.

Where is an OBD Port Located?

As stated earlier, an OBD port is usually in the driver’s footwell located in the vehicle’s dashboard. However, the exact position differs by the make and model of the car. For example, some cars have the OBD port. Others may secure it using a latch or compartment.

Regardless of where your vehicle’s manufacturer places the OBD2 port, it is readily accessible and doesn’t require any special tools. However, you may need to remove a small plastic protective covering to access its terminals.

What Does OBD Stand For?

OBDs were originally developed as a method of controlling vehicle emissions. However, their roles have expanded to diagnose a wide range of vehicle problems. They also monitor vehicle subsystems, and the more sophisticated ones can fix minor problems without human intervention.

Specific applications of OBDs include:

Vehicle emissions testing: While the initial iterations of OBD systems were largely unsuccessful in regulating emissions, modern OBDs have since overcome this limitation. They collect emissions-related trouble codes so inspectors can use diagnostic tools to check that a vehicle is compliant with local emission levels.
Fuel economy: OBDs can improve a vehicle's fuel economy by controlling the engine's fuel injectors. They can also regulate other electronic systems like air conditioning and climate control. If left unchecked, these systems can cause the engine to consume more fuel than normal.
Driver behavior monitoring: Advancements in sensors and ECU technology have made them sophisticated enough to monitor driver behavior. This feature helps drivers demonstrate safe driving practices, reducing their culpability during accidents or traffic violations. Moreover, companies can monitor their vehicle fleet thanks to data loggers to monitor drivers' real-time behavior.
Supplementary instrumentation: Modern OBDs have WiFi or Bluetooth integration that enables them to connect to scan tools remotely. Professional drivers and car enthusiasts with custom vehicle modifications can leverage this feature to view metrics not usually found in regular vehicles. Some OBDs can even connect to a smartphone with the appropriate app installed.

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OBD for Commercial Vehicles

There's hardly any industry that gets the most out of OBDs than the commercial vehicle industry. This is unsurprising, given that vehicle efficiency is the driving factor behind the success of this industry. In fact, OBD-II standards have special protocols and diagnostic trouble codes for commercial vehicles.

Commercial vehicle companies have developed advanced scan tools that provide more information than regular scan units. These tools help users access all of the following benefits of OBD systems:

Accurate diagnostics: Rather than relying on error-prone human technicians, OBD systems use an extensive network of sensors to collect vehicle information. The information received is very accurate and standardized, reducing the likelihood of missing or misinterpreting serious system errors.
Fast diagnosis: Using OBDs and scan tools to pull vehicle information is several times faster than manually inspecting the vehicle for problems. A repair technician can connect a scan tool to the OBD port within minutes using standard connectors (SAE J1962). The diagnostic information is compared to standard error codes for quick and accurate problem identification.
Improved compliance: OBD systems monitor emissions, carry weight, and other standards and ensure that vehicles meet the minimum requirements. They alert users when they violate compliance standards so the issue can be resolved quickly. Some OBDs are programmable to match location-based requirements.
Cost efficiency: All of the above benefits of OBD systems combine to reduce the cost of maintenance and repairs and improve fuel efficiency. They also help enforce safe driving, reducing the chances of an accident and all the legal troubles that come with it.
Additional telemetry: Modern OBD can collect much more information, the so-called fleet management software. Companies operating a fleet of vehicles use a Generic OBD-II to collect real-time information about each vehicle. Each OBD vehicle relays information about its location, monitors driver behavior, and assess vehicle compliance. Moreover, all this information can be broadcast over secure channels to remote observers. This allows companies and their drivers to follow legal requirements and company policies.

How Does an OBD Device Work?

In its simplest form, an OBD system consists of a central processor, a sensor network, a connection port, and several indicators. The sensors collect diagnostic data about the various subsystems within the vehicle and transmit it for processing to a readable format.

A standard OBD system contains these components performing the following functions:

ECU: The Electronic Control Unit is the central part of an OBD system. It gathers input from the sensors placed throughout the vehicle and processes the data to find issues. The ECU also uses the processed data to control other parts of the vehicle, like fuel injectors or air conditioning.
Sensors: Sensors collect analog and digital signals from all relevant parts of the vehicle, including the engine, chassis, and electrical system. They encode these signals with their source, strength, and other parameters. The data is then transmitted to the ECU for processing and interpretation.
DTC: Sensors are designed to detect signals within a preset range. If the signals fall outside the normal range, the ECU saves the information as a Diagnostic Trouble Code or DTC. DTC codes are standardized lists of letters and numbers that encode the type of problem and its source. Most DTC codes are platform-agnostic, but some may be specific to a manufacturer or vehicle model.

When the ECU generates a DTC, it sends a signal to the appropriate indicator light, alerting the vehicle’s passengers of a problem. Each problem can be one of four types, according to the DTC - P for powertrain (engine and transmission), B for body, C for chassis, and U for network. DTCs can also be pulled from the ECU through external computers and diagnostic devices connected to the OBD port.
MIL: Malfunction Indicator Lights (MILs) are turned on by the ECU when it receives a DTC code from sensors. Most MILs are located in the dashboard, but a few may be elsewhere in the vehicle. They serve as an early warning system for the vehicle, staying on for minor issues or flashing for problems that are more urgent.
DLC: The Diagnostic Link Connector or DLC serves as an interface between the vehicle's internal OBD system and other external systems. It allows these systems to access all the data and DTCs the ECU collects from sensors. It is often found underneath the driver's dashboard, but the actual location depends on the manufacturer. Most modern DLCs are made with a standard OBD II system for easy interface with type 2 OBD scanners.

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