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What does DTC mean?

Vehicles are advancing faster with the introduction of complex parts and the incorporation of computerised or automated options. Identifying problems in cars is also becoming easier since you don't have to understand the vehicle's anatomy. You can plug your vehicle into a computer and get a diagnostic report. Some cars also signal you through the dashboard screen when your car has a problem. Even older vehicles have service lights to signal you of issues present. When you know the DTC meaning, you can find a solution to your car's problem. Read on to discover ways to read DTC signals.

What is a DTC code?

During vehicle manufacture, engineers install European Onboard Diagnostic (EOBD) systems. The work of the EOBD is to monitor the car's performance, from the engine to emissions systems and gearbox. In case of a problem, it generates codes to indicate specific issues and turns on a dashboard warning. The code is the Diagnostic Trouble Code (DTC), accessible through a scanner or diagnostic tool connected to the EOBD socket. The light indicates a problem in your car and prompts you to connect the scanner to read the code.

DTC or engine fault codes come in five digits, including one letter and four numbers. Each letter and number combination relates to a vehicle's specific problem, from generic problems to manufacturer-specific errors. While a DTC code won't tell you the specific part that needs replacement or repairs, it informs you what you need to test to diagnose the problem.

In the past, vehicle manufacturers used different onboard diagnostics systems. Nowadays, you will find the OBD-II for reading codes in light and heavy-duty vehicles, while the J1939 is for heavy-duty vehicles or equipment. You will also find a DTC list with common codes relating to different car models. Vehicle manufacturers can also create specific DTC codes if their car model needs it.

Types of DTC codes

With the numerous manufacturer-specific and generic DTC codes, it is challenging to understand fault codes. Usually, the DTC meanings are in the manufacturer's manual. However, if you have difficulty understanding them, check with the mechanic before purchasing replacement parts. DTC codes are categorised into two types:

Critical codes

Critical codes indicate urgent DTC codes, which should be looked into immediately since they can cause severe damage to the vehicle. For instance, DTCs that indicate low coolant levels and high engine temperatures could signify that engine failure is imminent. Real-time diagnostics allows technicians to act quickly, assess the severity of the malfunctions and resolve them before they become costly and critical.

Non-critical codes

Non-critical codes don't require the same urgency as critical ones but need proper attention. They can include emission-related problems that should be fixed before they cause excessive pollution. Non-critical codes light up the malfunction indicator lamp or the check engine light.

Format of OBD-II diagnostic trouble codes

A DTC code has one letter and four numbers. For example, a code can be P0430. The code is divided into the following four sections:

dtc codes

System or category

The first letter of the code usually represents the category of the DTC and pinpoints the related system that requires attention. For the OBD-II, the diagnostic trouble codes are grouped into four categories or systems. If your DTC starts with B, it indicates a problem in the vehicle's body. The problem is found in the passenger compartment, like driver-related functions that improve safety and convenience. C codes indicate the chassis category, which means all areas outside the passenger compartment. Codes with the letter C cover problems in functionalities, including mechanical systems, steering, suspension and brakes. P codes are the powertrain category, covering problems in the transmission engine and drivetrain accessories. On the other hand, U codes cover network and vehicle integration functions managed by onboard computer systems. From the example above, DTC code P0430 shows that the car has problems with the powertrain system.

Generic and manufacturer specific codes

The second character in the DTC identifies if the code is generic or from the manufacturer. They usually start with 0 or 1. Codes that start with 0 indicate a generic fault, while those that start with 1 are manufacturer specific. If your code is generic, the vehicle has a problem common to all cars, while the manufacturer-specific problems are unique to the vehicle model. In the P0430 example, the code indicates a generic fault.


The third character in the DTC pinpoints the subsystem where the fault is located. The character digits range from 1 to 8 to indicate the specific problem:

• 1 shows a problem in the fuel or air metering system

• 2 refers to fuel and air metering injection

• 3 indicates the ignition system

• 4 relates to the emission system

• 5 indicates a problem with the vehicle speed controls

• 6 indicates an issue in the computer output circuit

The last two numbers, 7 and 8, show a transmission-related issue. Sometimes, the subsystems category shows letters A, B or C instead of numbers. The three letters indicate hybrid propulsion systems.

Fault code description

The last two characters in the DTC are actual fault description codes and can range from 0 to 99. Some fault code descriptions are also three numbers. From the fault description manual, you can pinpoint the specific problem. From the P0430 example, 30 shows that the catalyst system efficiency is below the threshold. When you understand the DTC meaning, you can check for problems like exhaust leaks that contribute to the signal.

How to interpret DTC meaning in J1939?

The J1939 DTC has four characters indicating faults in heavy-duty vehicles. The first letter is the Suspect Parameter Number (SPN) which indicates the error in the vehicle. The Failure Mode Identifier shows the nature and type of error, from calibration errors to sensor short circuits and incorrect update rates. The Occurrence Counter records the SPN and stores information on the error even when the error is inactive.

How to extract DTC fault codes?

If you want to read diagnostic trouble codes, you need a DTC scanner and a diagnostic connector. Some DTC readers show the character codes without descriptions, while others will interpret the codes. Diagnostic connectors found in your car differ depending on the vehicle model. If your car was made before 1996, you need specific adapters to connect with the connector. The latest models have a universal adapter and similar connectors.

When extracting a DTC code for interpretation, you start by plugging a scanner or code reader into the OBD-II diagnostic connector. You will find the connector under the dashboard left of the steering column. The scanner communicates with the electronic control unit when you turn on the ignition. Access the tool's menu and click the read codes option from your scan tool. The scanner will automatically detect the vehicle's model and year. If it doesn't, you have to enter the specifics manually. After revealing the car model, you can see the fault codes from the scanner. You can transfer the DTC codes to your computer or phone using Wi-Fi or Bluetooth options. The DTC codes provide insights into the possible vehicle problem. If you don't know how to read the codes, consult a mechanic before diagnosing and fixing the malfunction.

When buying DTC scanners for reading fault codes, you can choose from handheld or wireless options. Handheld EOBD scanners come with a cable to plug into the car's connector and a screen for displaying the codes. Wireless scanners plug into the port, but you can read the code from your smartphone through Wi-Fi or Bluetooth connectivity. You should get an EOBD-II scanner that displays both the code and the meaning to avoid referring to the car manual every time you check for faults.

Clearing DTC codes

If you want to clear DTC fault codes from the car, you need to fix the malfunction, and the code will disappear. The check engine light will not stop displaying until you fix the problem. In previous versions of OBD, a driver could clear the DTC fault codes and prevent the car from displaying warnings. To remove the DTC in previous OBD models, drivers disconnected the battery or power source, clearing the computer's temporary memory. The check engine warning light would turn off, and you could continue with your journey without fixing the issue.

The latest OBD-II versions store the DTCs generated in non-volatile memory. Even if you disconnect the power source, the fault codes are intact until you fix the problem or clear them using a scanner. Clearing your DTCs by disconnecting the battery can lead to electronic radio reset and climate control system loss. You can also lose the engine's learned memory, which is difficult to recover. The engine's computer system usually contains adjustments made over time to compensate for engine wear and your driving habits.


Understanding the DTC meaning is important since the code won't specify the source of the problem. If you can interpret the fault codes, you can make the correct diagnosis, find replacements or do the necessary repairs.


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