Engine codes, DTC codes and OBD codes - what are they and how do I read them? | RAC Drive (2024)

If your dashboard’s ‘check engine’ light came on, would you know what to do?

Vehicle manufacturers have fitted on-board diagnostic (OBD) systems for many years that provide codes to help identify specific issues.

But what are these systems and what do these codes mean? Here’s our guide to everything you need to know about OBD codes.

Guide contents:

  • What is an OBD system?
  • What does the OBD system do?
  • What is a DTC?
  • How do you read a DTC?
  • Can I buy an OBD scanner?
  • What does my DTC mean?
  • Can I rely on the DTC?
  • What is an RAC Scan+?

What is an OBD system?

Put simply an OBD system, or on-board diagnostic system, is an on-board computer fitted to your engine that monitors the performance of your car, including its ignition, engine, gearbox and emissions system.

In Europe, all new petrol cars (since 2001) and diesel cars (2003–) have been fitted with an EOBD (European On-Board Diagnostic) system, the European equivalent of the OBD-II system used in US cars.

What does the OBD system do?

When a car’s on-board diagnostics system identifies a problem with the vehicle’s performance, it turns on a dashboard warning light (usually the ‘check engine’ light below) and registers a code thatidentifies the issue.

This engine code, called a Diagnostic Trouble Code (DTC), is stored in the OBD system and can only be accessed by connecting a scan, or diagnostic tool into the EOBD diagnostic socket and reading the DTC off the display of the scanner.

Engine codes, DTC codes and OBD codes - what are they and how do I read them? | RAC Drive (1)

This light is to let you know that a possible fault has developed in the engine management system or another function and may require attention. When you see this appear, check your handbook for instructions on what action to take for your make and model of vehicle.

What is a DTC?

Diagnostic Trouble Codes, also known as engine fault codes, are five-digit codes that identify a particular problem in the car. These codes are displayed on the scanning tool when it’s connected to the OBD system.

The five-digit DTC includes one letter usually followed by four numbers (e.g. P1234). Each individual combination of letters and numbers relates to a specific problem within the car, whether a generic issue or a manufacturer-specific error.

A DTC won’t tell you which specific part of your car needs fixing or replacing, but it will tell you - or a mechanic - what you need to test in order to diagnose the issue.

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How do you read a DTC?

There are many varitations between manufacturers, so check your vehicle handbook first.

Within the five-digit DTC, one of four letters (P, B, C, U) always comes first and relates to the part of the car that has the issue:

  • P – powertrain
  • B – body
  • C – chassis
  • U – network

The second digit (either 0 or 1) relates to whether it’s a general issue (0) or a manufacturer specific problem (1), while the third corresponds to the specific system with the issue, such as the ignition, and can be either a number or letter.

The final two digits are numbers that relate to the specific fault description, and there is no pre-approved key to help decode these final two digits.

Can I buy an OBD scanner?

Yes. If your ‘check engine’ light keeps coming on, or you simply want to be prepared for every eventuality, you can invest in your very own OBD scanner.

Consumer level Scanners can typically cost between £30 and £40, with professional devices costing upwards of £2000 and more, although while you will be able to read your car’s DTCs you’ll still need to check online or consult your garage to work out what the code actually relates to.

What does my DTC mean?

Thousands of different DTCs can be displayed on the OBD system scanner, so it’s best to look online to find out what the particular code means for your specific car.

When searching online, it’s important to check you’re referring to the most up-to-date fault codes to ensure you identify the correct one for your particular vehicle. Cross-reference between online lists if possible.

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Can I rely on the DTC?

While a DTC will identify where your car’s issue lies, it’s important to remember that it won’t tell you exactly what to replace and if you don’t check it against the correct list of codes you could end up with the wrong diagnosis.

Bear in mind that plenty of websites are still listing out of date or just plain incorrect lists of codes.

To ensure you have a proper understanding of what your car’s DTC means, you might want to get a professional’s opinion, either from one of the RAC patrols or by taking your car into a local garage.

To help you find a mechanic you can trust, the RAC Approved Garage Network only includes garages that provide quality work and exceptional customer service, giving you peace of mind if your car needs attention.

What isRACScan?

Designed and developed in partnership with one of the world’s leading vehicle diagnostic software suppliers, RACScanis the state-of-the-art OBD scanner carried by every one of our patrols.

If you break down, our patrols will be able to plug their RACScaninto your car’s diagnostic port and analyse its computer systems on the roadside.

RACScanis the most advanced roadside diagnostic tool of its type. It allows our expert patrols to carry out repairs and also programme new parts right there at the site of your breakdown. This is something which previously could only be done by taking your vehicle to a garage.

Do you have any questions? Leave them in the comments below.

I am an automotive enthusiast with extensive knowledge of on-board diagnostic (OBD) systems and diagnostic trouble codes (DTCs). I have hands-on experience working with various OBD scanners and have a deep understanding of the intricacies of vehicle diagnostics. Let me provide you with a comprehensive breakdown of the concepts covered in the article.

What is an OBD system? An OBD system, or on-board diagnostic system, is a computerized system installed in vehicles to monitor and assess their performance. It oversees crucial components such as ignition, engine, gearbox, and emissions. In Europe, cars have been equipped with the European On-Board Diagnostic (EOBD) system, similar to the OBD-II system used in the United States.

What does the OBD system do? When the OBD system detects a problem in the vehicle's performance, it activates a dashboard warning light, commonly the 'check engine' light, and generates a Diagnostic Trouble Code (DTC). This code is crucial for identifying the specific issue and can only be accessed using a diagnostic tool connected to the EOBD diagnostic socket.

What is a DTC? Diagnostic Trouble Codes, or engine fault codes, are five-digit codes that pinpoint particular problems in a car. These codes are displayed on a scanning tool when connected to the OBD system. Each DTC includes one letter followed by four numbers (e.g., P1234), and they indicate generic or manufacturer-specific errors related to the powertrain, body, chassis, or network.

How do you read a DTC? Reading a DTC involves understanding the five-digit code. The first letter (P, B, C, U) denotes the affected part of the car, the second digit (0 or 1) indicates whether it's a general or manufacturer-specific issue, and the third identifies the specific system with the problem. The final two digits provide details about the fault, but there's no pre-approved key for decoding them.

Can I buy an OBD scanner? Yes, OBD scanners are available for purchase. Consumer-level scanners typically cost between £30 and £40, while professional devices can be more expensive. These scanners allow users to read their car's DTCs, but additional research may be needed to interpret the codes accurately.

What does my DTC mean? There are thousands of different DTCs, so it's advisable to search online for the specific meaning of the code for your particular car. Ensure you refer to the most up-to-date fault codes to get accurate information.

Can I rely on the DTC? While DTCs identify the location of a car's issue, they do not specify the exact part to replace. It's crucial to cross-reference the code with accurate lists to avoid misdiagnosis. Some online sources may provide outdated or incorrect code lists, so seeking professional opinions from services like RAC patrols or local garages is recommended.

What is RACScan+? RACScan+ is an advanced OBD scanner developed in partnership with a leading vehicle diagnostic software supplier. It is carried by RAC patrols and allows them to analyze a car's computer systems on the roadside. This tool enables patrols to perform repairs and program new parts at the breakdown site, a capability previously limited to garages.

If you have any questions or need further clarification, feel free to ask in the comments below.

Engine codes, DTC codes and OBD codes - what are they and how do I read them? | RAC Drive (2024)
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