Diagnostic Trouble Code: How They Work + How to Identify Them

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Diagnostic Trouble Code

Diagnostic Trouble Code – When the diagnostic system installed on a vehicle detects a fault, an appropriate diagnostic code will be generated, generally notifying the driver via a warning light or other indicator on the vehicle’s instrument panel.

This diagnosis code is often referred to as a Diagnostic Trouble Code (DTC). This DTC code is useful to help you understand what needs to be repaired so that the vehicle continues to operate properly and safely. In this article, we will outline the concept of Diagnostic Trouble Codes (DTC) along with methods for interpreting them. Get to know more in this TransTRACK article!

What is a Diagnostic DTC?

DTC stands for Diagnostic Trouble Code. DTC codes refer to a series of diagnostic trouble codes used by the vehicle’s onboard diagnostic system (OBD) to notify you when your vehicle is malfunctioning. Each code has a different DTC interpretation and reflects a specific problem that occurs with your vehicle.

If the vehicle’s OBD system detects a failure, a certain DTC code will be generated and sends an alert via a warning light on the vehicle’s instrument panel. On vehicles equipped with telematics technology, these alerts can be relayed directly to the fleet. The system can be configured to send alerts directly to maintenance.

The code was created by the Association of Automotive Engineers (SAE) with the goal of helping vehicles meet emissions regulations. Today, SAE is known as SAE International, and is a professional body that develops standards for engineers in the automotive field.

List of Standard Diagnostic Trouble Codes

Today’s vehicles are equipped with internal computers that not only monitor the overall performance of the car, truck, or SUV, but also provide notifications about problems by generating trouble codes. Of course, if your vehicle’s computer generates the code, you certainly need to understand the meaning of the trouble code.

Before understanding what each code means and how to work around it, you need to understand two important concepts.

  • The diagnostic system embedded in the vehicle computer is an integrated device that monitors various important systems in your vehicle. OBD can provide warnings of potential problems with the vehicle’s engine, airbags, anti-lock brakes (ABS), and other components. Since 1996, vehicles have been equipped with an OBD-II diagnostic system. If your vehicle is equipped with this system, an OBD-II diagnostic scan tool is required to interact with the OBD system.
  • A DTC is a series of codes generated by your vehicle’s OBD to tell you that something is wrong. By reading a combination of five letters and numbers, a DTC (Diagnostic Trouble Code) allows you to enter a system that is experiencing problems. For example, if your vehicle’s OBD detects a leak in the evaporative emissions system, you may encounter code P0442. In general, today’s vehicles have hundreds of possible DTC codes that the car can generate.

DTC Code Type

These numbers or codes are created by the vehicle’s integrated diagnostic (OBD) system. Each digit in a 5-digit Diagnostic Error Code (DTC) represents a specific problem. There are two main types of code that apply:

Category 1: Critical Code

This is a critical DTC Code and can cause rapid and serious damage to the vehicle.

DTC codes such as a high engine temperature indicator and a low coolant level can indicate that engine damage may occur in the near future. With the help of real-time diagnostic systems on vehicles, fleet managers and technicians can immediately take necessary steps.

They can evaluate the severity of the situation, provide recommendations for appropriate action, and locate the nearest repair shop to resolve the problem before it transforms into a costly and critical situation.

Category 2: Non-Critical Code

Although this DTC code does not require immediate resolution, it still requires proper attention. This involves including engine-related DTCs that are emissions-related and do not impact excessive pollution.

Non-critical codes include codes that activate the Malfunction Indicator Light (MIL), also known as the Check Engine Light. This type of code also records freeze failure in two consecutive driving periods after two driving cycles.

How DTC Codes Work

Every time a problem occurs in your car or an error is detected, the OBD system in your vehicle will generate a Diagnostic Trouble Code (DTC).

Once detected, the OBD system will warn you through the appearance of a DTC code, which is usually indicated by the ignition of the Malfunction Indicator Light (Check Engine Light).

DTC codes allow external devices, such as car-mounted diagnostic scanners (code readers), to communicate with your car’s OBD system.

Initially, there were several types of diagnostic interfaces installed in cars, which varied depending on the vehicle manufacturer.

These codes play a role in ensuring that cars meet emissions regulations and contribute to environmental protection.

How to Analyze the Diagnostic Trouble Code

Don’t let OBD codes and their variations worry you. All you need to do is understand the code arrangement of Diagnostic Trouble Codes (DTC).

This layout is relatively simple, with five characters for each DTC code. Let’s go through the OBD code structure character by character:

First Character of DTC Code

  • The first character of the DTC indicates the affected system:
  • Code B: Body
  • Code C: Chassis (suspension, braking, steering, etc.)
  • P Code: Powertrain (fuel system, engine, drivetrain, transmission, etc.)
  • U code: Onboard computer and function integration by OBD.

Second Character of the DTC Code

  • The second character indicates whether it is an international generic SAE code or a manufacturer-specific code.
  • 0: Usually the second character is 0. All vehicles that follow the international OBD-II standard have 0 in the second character.
  • 1-9: If the second character is a number 1-9, it is best to contact the vehicle manufacturer for further information.

Third Character DTC Code

  • If the second character of the code is 0, then the third character indicates the subsystem involved.
  • 0-2: Issues with fuel and air gauges
  • 3: Ignition-related issues
  • 4: Additional emission controls
  • 5: Vehicle speed control and idle system issues
  • 6: Computer related problems 7-8: Difficulty with transmission


Diagnostic trouble codes have a central role in opening up horizons to potential problems. Understanding the essence of Diagnostic Trouble Codes empowers vehicle owners and technicians to resolve issues with efficiency. By interpreting DTC codes accurately, you can ensure your vehicle remains in optimal condition and avoid more complex and costly repairs in the future.

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