#OBD Codes

OBD Diagnostic Trouble Codes (DTC)

OBD Diagnostic Trouble Codes (DTC)
Modern cars are powered by a complex control unit network that manages everything from engine performance to safety features. These units communicate with each other, making sure your vehicle runs efficiently and safely. 
Today's cars also have a system called on-board diagnostics, or OBD for short. It constantly checks these control units to ensure they're in good shape. If there's a problem, the OBD system flashes a code to pinpoint what and where the issue is. 
Understanding what these codes mean is crucial so you can diagnose and fix car issues quickly. 
That's where our guide comes in. Here, we'll explain all about OBD codes, break down the common and not-so-common faults, and give you troubleshooting tips. 

What is On-Board Diagnostics (OBD)?

On-Board Diagnostics, also known as OBD, is an electronic system in vehicles used for self-diagnostics and issue reporting. It uses data from various sensors to monitor the vehicle's performance. When OBD detects a problem, it generates a trouble code or, for more serious issues, triggers a dashboard alert
Initially, the automotive industry used OBD-I, the first generation of these diagnostic systems. OBD-I was quite basic, and its functionality varied significantly from one car manufacturer to another, making standardized diagnostics a challenge. 
Transition to OBD-II 
The evolution to OBD-II (often called OBD2) represented a step forward in diagnostic technology. Initially implemented in the United States, OBD2 introduced a uniform protocol that applied across all car brands. This simplified the diagnostics process, allowing for the same servicing and repair tools and procedures for any vehicle. 
European On-Board Diagnostics (EOBD) 
The European On-Board Diagnostics (EOBD) regulations are the European counterpart to OBD2. Developed to comply with the European Union's stricter emission regulations, EOBD monitors vehicles to ensure they meet these standards. 
The importance of standardized diagnostic 
Systems like OBD2 and EOBD have significantly changed how we approach car maintenance. They make diagnosing and fixing cars much easier, streamlining the process for mechanics and car owners.  
Standardized diagnostics has two main goals: 
First, it simplifies identifying and solving car problems. This means less guesswork and more accurate fixes.  
Second, it monitors car emissions. Tracking emissions ensures vehicles meet environmental standards, so they’re less harmful to our planet. 
 ChatGPT A person in an Audi holding a smartphone with the OBDeleven car diagnostic app displaying vehicle diagnostics and fault codes.

What is an OBD code?  

OBD codes, or Diagnostic Trouble Codes (DTCs), are the method through which a vehicle communicates issues detected by its onboard computer system. Each OBD code corresponds to a specific problem and helps identify and diagnose malfunctions within the vehicle's systems. 
Broadly, OBD codes fall into two groups:  
  • Standard OBD2 trouble codes that can be accessed using any OBD2 reader 
  • Manufacturer-specific trouble codes, which may require specialized equipment to interpret. 
What is a standard OBD2 trouble code? 
Standard OBD2 trouble codes are diagnostic codes used across all car brands and models equipped with an OBD2 system. 
These codes typically begin with a letter followed by four digits (e.g., P0301). The initial letter indicates the system related to the fault (e.g., P for Powertrain), while the digits specify the exact issue. 
When the check engine light illuminates, it often means that one or more OBD2 trouble codes have been generated. This alerts the driver to perform a diagnostic check to identify the issue. 
Note: The term "OBD2 codes" is often used to refer to all diagnostic trouble codes in general. However, in this article, we use this term to refer only to the generic trouble codes listed in the SAE J2012 standard, and not the manufacturer-specific codes. 
What is a manufacturer-specific trouble code? 
While OBD2 codes are universal, manufacturer-specific codes give more detailed information about problems unique to certain car models. These codes help manufacturers identify issues not covered by the standardized OBD2 system. 
Like standard OBD2 codes, manufacturer-specific codes often start with a letter that signifies the system category. However, the meaning of the subsequent digits is determined by the manufacturer. These codes can reveal more about the car's condition, especially for complex systems not addressed by the generic fault codes. 
In essence, all OBD codes, whether they're standard OBD2 trouble codes or manufacturer-specific, serve as a crucial tool in diagnosing and addressing vehicle issues. By interpreting these codes correctly, mechanics and car owners can understand precisely what's going wrong with the vehicle. 

How are OBD codes formed? 

Like we said, OBD codes are formed by a sequence of letters and numbers, each providing specific information about the issue detected in the vehicle. Let's break down what each part of the OBD code means. 
What does the first character of the OBD fault code mean? 
The first character in an OBD fault code is a letter that identifies the affected car system: 
  • P (Powertrain) – engine or transmission 
  • C (Chassis) – chassis category, which includes the frame, steering, and suspension systems 
  • B (Body) – issue with body-related parts like airbags or seat belts 
  • U (User Network) – network or communication errors between the car's electronic systems
What does the second character of the OBD fault code mean? 
The second character is a digit that tells us about the nature of the code: 
  • 0 – a standard code that follows the universal OBD2 system 
  • 1 – a manufacturer-specific code that's unique to the vehicle’s make 
  • 2 – in the 'P' category, it’s a standard code. In other categories, it's manufacturer- specific code. 
  • 3 – in the 'P' category, it’s a jointly defined code. Not used in other categories yet. 
What does the third character of the OBD fault code mean?  
The third digit narrows the code down to the subsystem involved: 
  • 0, 1, and 2 – fuel and air metering systems 
  • 3 – ignition system or misfires 
  • 4 – auxiliary emissions controls 
  • 5 – vehicle speed and idle control systems 
  • 6 – on-board computer and auxiliary outputs 
  • 7, 8, and 9 – transmission 
  • A-F – hybrid system 
What do the fourth and fifth characters of the OBD fault code mean?  
The final two digits, ranging from 00 to 99, specify the exact problem. The codes can refer to a long list of potential issues. Diagnostic tools and apps are designed to interpret these codes and explain the fault for proper troubleshooting. 
Understanding how OBD codes are structured can give you insights into the nature of the problem your vehicle is experiencing, which is especially useful before running a detailed diagnostic. 

How to read trouble codes?  

To read trouble codes, you’ll need some equipment. The two main tools for this job are code readers and scan tools, each with different capabilities. 
OBD2 code reader
The OBD2 code reader is a relatively straightforward tool. It’s designed to plug into your car's OBD2 port and read the fault codes that have been triggered. This tool will display the codes and can sometimes clear them from the system after the issue has been fixed. It's a handy device for a quick check. 
OBD scan tool
An OBD diagnostic scanner does everything a code reader does and more. It can read various codes, including pending, generic, and manufacturer-specific ones. 
Beyond just reading and clearing codes, it may provide troubleshooting tips and freeze frame data to help diagnose the problem further. Advanced scan tools can even show and record live data values from your vehicle and display them in charts. 
Code reader vs scan tool
The key difference between a code reader and a scan tool lies in the level of detail and functionality: 
  • Code readers are great for a quick diagnosis and are usually more affordable. 
  • Scan tools offer comprehensive data and troubleshooting features, making them better suited for detailed diagnostics and professional use. 
When the check engine light comes on, both tools can help pinpoint the issue. However, an OBD scanner can provide a deeper analysis of more complex issues. 

How to clear trouble codes?  

Once you've identified the trouble codes and fixed the indicated issues, you can reset these codes from the car's memory. There are a few ways to do this: 
Clearing codes with an OBD scanner 
The most straightforward method to clear trouble codes is by using an OBD scan tool. After repairs have been made to address the issues that triggered the codes, you can use the scan tool to reset the system. This usually involves navigating the tool's menu and selecting the option to clear or reset the codes. 
Resetting codes without a scanner 
If you don’t have a scanner, you can reset the codes by disconnecting the car battery. Turn off the car, detach the negative battery terminal, wait a few seconds, and reconnect it. This can reset the computer system and clear the codes or make them inactive. However, it’s less reliable and can disrupt other electronic settings. 
Fixing the issue and waiting 
Sometimes, if you've fixed the underlying problem, the trouble codes will reset on their own after some driving cycles. Suppose the system checks the problem and finds it resolved across several cycles – in this case, it may automatically turn off the dashboard lights and make trouble codes inactive. 
Note: Just clearing the codes won't fix the problem. Proper diagnostics and repairs are necessary to ensure the vehicle runs correctly and safely. 

How long are OBD diagnostic trouble codes stored?  

OBD diagnostic trouble codes are stored until someone manually clears them using a scan tool or the vehicle's onboard computer system automatically erases them after the issue has been fixed for a certain period of time. 
The unlearning counter 
The unlearning counter monitors the number of drive cycles completed without the fault reoccurring. Each fault code has its own unlearning counter, which counts the number of times the vehicle starts and operates without the problem arising again.  
When this counter reaches a certain threshold set by the manufacturer without further issues, the code is automatically cleared from the memory. This ensures that the vehicle’s system holds only relevant data. 
Note: The exact number of drive cycles needed for the code to be cleared can vary from a few dozen to several hundred cycles. It usually depends on the specific code. 

Common OBD codes  

Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.

Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.