What Are DTC Codes: Types and How to Read Them

If you have ever been stranded on the road because of a mechanical issue with your vehicle, you know never to ignore a car warning light. This is even more important when managing a fleet because mechanical issues can lead to delays, which affects customer satisfaction.

When your vehicle's on-board diagnostic system detects an issue, it will generate a DTC code and alert the driver through a warning light. The code tells you where the problem is so you can fix it. Are you wondering how to read DTC codes? Read on to learn more. However, let's first understand what DTC codes are.

What are DTC codes?

DTC codes are short form for Diagnostic Trouble Codes. These are standardized codes used to identify and communicate issues detected by a vehicle's on-board diagnostic system. The Society of Automotive Engineers (SAE) developed them to troubleshoot problems related to the engine, transmission, brakes, and other critical systems. When something goes wrong, the OBD system stores a specific DTC in its memory and will trigger the "Check Engine" light or other warning indicators on the dashboard.

The use of diagnostic codes in vehicles dates back to the late 20th century. However, it wasn't until 1996 that DTC codes became standardized with the implementation of the OBD-II system. OBD-II, short form for On-Board Diagnostics II, is a standardized system mandated by the United States Environmental Protection Agency(EPA) for all cars and light trucks sold in the United States starting from 1996. The primary goal was to reduce emissions and provide a universal method for diagnosing vehicle issues.

A DTC comprises a combination of letters and numbers. All of them together tell you what the problem is and where. Want to know how to read DTC codes? Continue reading.

Types of DTC codes

DTC codes are put into two categories: generic and manufacturer-specific.

Generic

These codes are common across different vehicle makes and models. They provide a general indication of the problem, such as a misfire or sensor malfunction. Generic codes are further divided into two:

• OBD-II codes: These are used in light and medium-duty vehicles. All cars manufactured and sold in the US since 1996 follow this standard.
• J1939 codes: These are used for heavy-duty vehicles and heavy equipment.

Manufacturer-specific

These codes are unique to a particular vehicle manufacturer. They offer more detailed information about an issue, often related to specific components or systems within the vehicle.

OBD-II codes

OBD-II codes and DTC codes are often used interchangeably. While they refer to the same thing, there's a slight difference in emphasis. OBD-II stands for on-board diagnostics II. It is the standardized system used in most vehicles manufactured after 1996. This system monitors and controls various engine and vehicle functions. OBD-II codes are a subset of DTC codes that adhere to the OBD-II standards.

When a code is generated, you will use an OBD-II code reader to read the code. The code will tell you the exact source of the problem.

Types of OBD-II codes

OBD-II codes are categorized into several types based on the different systems they monitor and the specific nature of the issues they report. These codes are structured to allow technicians to pinpoint problems within a vehicle's systems. Each OBD-II code consists of five characters, with the first character indicating the type of code. Here are the main types of OBD-II codes:

Powertrain codes (P-codes)

Powertrain codes relate to the engine, transmission, and emissions systems. They are by far the most common type of OBD-II codes because the powertrain is crucial for both vehicle performance and emissions control. These codes cover issues such as fuel system malfunctions, ignition problems, and sensor failures.

Let's look at this powertrain code: P0101. The code shows a problem with the Mass Air Flow (MAF) sensor. The MAF sensor calculates the amount of air getting into the engine, enabling the ECU (engine control unit) to calculate and adjust the proper air-fuel ratio mixture for optimal combustion and performance. When the MAF fails, you could have issues with engine performance and fuel efficiency.

Body codes (B-codes)

Body codes are concerned with the various electronic systems related to the vehicle's body. These codes cover issues with systems such as airbags, climate control, lighting, and central locking. B-codes are less common than P-codes, but they are important for diagnosing non-engine-related problems that can affect the safety and comfort of your car.

For instance, a B0020 code points to a problem with the passenger's side airbag deployment circuit, while B1231 indicates the crash event memory is full and locked. On the other hand, B1342 shows that the electronic control unit (ECU) is defective or malfunctioning.

Chassis codes (C-codes)

Chassis codes deal with chassis-related systems, including anti-lock braking systems (ABS), traction control systems, electronic stability control, and suspension systems. These codes are crucial for diagnosing issues that can affect vehicle handling and braking performance.

For example, C0035 indicates an issue with the left front wheel speed sensor circuit. A faulty sensor, damaged wiring, or a poor connection in the circuit could cause the issue. This typically affects systems like ABS and traction control, which rely on accurate wheel speed data. C0242 shows a problem with the ABS control system's power relay chassis ground circuit, and C1234 indicates an erratic or intermittent signal from the rear right wheel speed sensor.

Network & undefined codes (U-codes)

Network and undefined codes pertain to communication problems between different modules within the vehicle. Modern vehicles have a multitude of electronic control modules (ECMs) that communicate over a network, often using the Controller Area Network (CAN) protocol. U-codes identify issues such as lost communication or network faults between these modules.

For example, U0100 indicates a loss of communication with the Engine Control Module (ECM) or Powertrain Control Module (PCM). It means that the vehicle's communication network is unable to establish or maintain a connection with the ECM/PCM, which can lead to various performance issues.

U0121 signals a loss of communication between the vehicle's primary network and the ABS control module. At the same time, U1000 shows a malfunction in the Class 2 serial data communication line, which is used for data exchange between various vehicle modules.

How do you read DTC codes?

DTC codes comprise five letters and digits, each conveying a different meaning.

First letter: Trouble code system

Refer to the system related to the fault.

• P: Powertrain
• B: Body
• C: Chassis
• U: Network & Undefined

Second digit: Code type

Indicates whether the code is generic or manufacturer-specific.

• 0: Generic
• 1: Manufacturer-specific

Third digit: Affected system

Refer specifically to the affected part.

• 1: Secondary air injection system
• 2: Fuel system
• 3: Ignition system
• 4: Exhaust monitoring systems
• 5: Cruise control
• 6: Computer auxiliary outputs
• 7,8: Transmission systems

Fourth and fifth digit: Specific code

Provide specific information about the fault, such as the exact sensor, circuit, or component that is malfunctioning. For example, the '20' in code "P0420" shows a problem with the catalytic converter's ability to effectively reduce engine emissions.

How to read DTC codes

Acquire an OBD-II scanner

To read DTC codes, you will need an OBD-II scanner or code reader. These devices come in various forms, from basic code readers that provide a simple readout of the codes to advanced scanners that offer live data, freeze frame data, and the ability to clear codes.

Locate the OBD-II port

The OBD-II port is usually located under the dashboard, near the driver's side. It's a 16-pin connector that allows the scanner to communicate with the vehicle's ECU.

Connect the OBD-II scanner

Plug the OBD-II scanner into the port. Turn on the car's ignition without starting the engine to allow the scanner to communicate with the ECU.

Retrieve the codes

Follow the OBD-II scanner's instructions to retrieve the stored DTCs. Some scanners may also provide definitions of the codes, while others may require you to look up the meaning separately.

Interpret the codes

Use the structure of the code to interpret what it means. For example, a P0302 code indicates a misfire in cylinder 2. You can refer to the scanner's database or a DTC code chart for more detailed information.

Diagnose and repair

Once you have identified the codes, you can begin diagnosing the problem. This may involve checking the related components, sensors, wiring, and even the ECU itself. If you don't have much knowledge about car repair, it's better to take the car to a mechanic for diagnosis and repair.

Clear the codes

After diagnosing and repairing the issue, use the scanner to clear the DTCs from the ECU. If the problem is not resolved, the code may reappear after a short drive cycle.

Tips and best practices when diagnosing DTC codes

When working with DTCs, you want to get as much information as possible so you can diagnose and fix the problem. For best results, follow the tips and best practices below.

• Use a reliable OBD-II scanner.
• Understand what each code means so that you can get an accurate diagnosis.
• Take note of any related codes.
• Do a visual inspection to identify any other issues with the car, even if they are not associated with the DTC you received. It can help you narrow down the problem.

Summing it up

DTC codes are an integral part of automotive maintenance and repair. They help diagnose problems and also play a role in preventive car maintenance. As an experienced driver or someone looking to buy a car, you need to learn how to read and interpret DTC codes. This can save you from being stranded on the road because you received a warning light but didn't act on it.