A Complete Guide to Understanding DTC Codes

Key Takeaways:

• DTC codes are error messages from a vehicle that show
• DTC codes use sensors to find problems and create codes that explain the issue.
• Understanding DTC codes helps you decide how urgent a repair is and keep your vehicle running smoothly.
• Fleet telematics systems make finding and fixing problems quickly easy, saving time and money. 

What are DTC codes?

DTC codes (Diagnostic Trouble Codes), also known as engine fault codes, are errors issued by a vehicle’s onboard diagnostics system (OBD). They are usually helpful in diagnosing engine, brake, or electrical faults.  

DTC codes are a detailed version of the “check engine light.” They give specific information about what’s wrong and where to look.

Fleet managers must understand DTC codes, as this enables faster troubleshooting, reduces maintenance costs, and ensures better vehicle availability.   

How Do DTC Codes Work?

When errors are detected by a vehicle’s onboard diagnostics system (OBD) in one of its monitored components, the system tracks data from sensors across the vehicle, checking for irregularities or malfunctions. If a fault is detected, the OBD system generates a specific code that pinpoints the problem.

Each DTC is tied to a unique fault, indicating what went wrong and where it occurred. These codes can be accessed through warning lights, like the “check engine” light, or by connecting a diagnostic scanner to the vehicle’s OBD port.

Modern vehicles rely on two primary OBD standards to generate and read DTC codes:

• OBD-2: Common in light and medium-duty vehicles.
• J1939: Designed for heavy-duty trucks and equipment.

These standards streamline diagnosing faults, making it easier for fleet managers and technicians to address issues quickly. While generic codes are easy to interpret, manufacturer-specific codes may require expert knowledge for accurate troubleshooting. 

Types of DTC Codes 

Aspect	Critical DTC Codes	Non-critical DTC Codes
Meaning	Indicate faults that may cause immediate and severe vehicle damage.	Refer to situations that do not demand immediate care, but still require attention.
Examples	High engine temperatures, low coolant levels, risk of engine failure, etc.	Emission-related engine problems, excessive pollution, etc.
Importance	High	Moderate
Response Time	Immediate action is critical; contact local service facility promptly.	Timely attention is necessary to maintain environmental standards and vehicle performance.
Risk Assessment	Signifies urgent issues that can lead to costly and urgent problems if ignored.	Can lead to environmental harm or gradual deterioration of vehicle performance if not addressed.
Potential Consequences	Severe vehicle damage or failure, safety risks.	Environmental pollution, decreased performance over time.
Recommended Actions	Act quickly, seek professional service immediately.	Address the issue in a timely manner, schedule maintenance as soon as possible.

Understanding the Structure of a DTC Code

A DTC code comprises five characters, each providing valuable information about your vehicle’s condition. With advanced fleet management solutions, the meaning of each code can be automatically decoded, helping identify the specific area of the car that needs attention. Here’s a breakdown of the code structure:

The First Character

The first character is a letter that identifies the part of the vehicle with the fault:

• P codes: Powertrain-related issues like engine, transmission, or fuel system problems.
• C codes: Chassis-related problems involving steering, suspension, or brakes.
• B codes: Body-related issues within the passenger compartment, such as airbags or HVAC systems.
• U codes: Problems with onboard computers or communication networks.

The Second Character

The second character is a number, either 0 or 1, which specifies the type of code:

• 0: The vehicle follows the OBD-II international standard.
• 1: Manufacturer-specific code; contact the manufacturer for details.

The Third Character

If the second character is “0,” the third character identifies the malfunctioning subsystem:

• 0: Fuel and air metering, auxiliary emission controls.
• 1: Fuel and air metering.
• 2: Fuel and air metering – injector circuit.
• 3: Ignition systems or misfires.
• 4: Auxiliary emission controls.
• 5: Vehicle speed, idle control systems.
• 6: Computer and output circuits.
• 7: Transmission.

For example, this character specifies the affected subsystem in a powertrain fault.

The Fourth and Fifth Characters

The last two characters of a DTC code are numbers that define the exact problem your vehicle is experiencing. These numbers range from 00 to 99.

The best resource for understanding DTCs is the dealer or manufacturer of the vehicle. Some codes are specific to the vehicle, so accessing a complete list or using a diagnostics software solution can help provide definitions.

For example, code P0442 indicates a small leak in the evaporative emission system. 

Common DTC Code List

Code	Fault
P0101	Mass Air Flow (MAF) circuit or sensor fault
P0110	Malfunction of intake air temperature sensor circuit
P0442	A small system leak within the evaporative emission control system
P0500	Malfunction in the speed sensor
P0606	A PCM (or ECM) powertrain malfunction
P0706	A transmission range sensor circuit range fault

How to Interpret a J1939 DTC Code?

J1939 DTC codes help identify faults in heavy-duty vehicles. Each code consists of these key parts:

• SPN (Suspect Parameter Number): Identifies the system or component with the problem.
• FMI (Failure Mode Identifier): Explains the type of fault, like high voltage (FMI 3) or an open circuit (FMI 5).
• CM (Conversion Method): Shows how the SPN is formatted or communicated.
• OC (Occurrence Count): Tells how often the fault has happened.
• SA (Source Address): Identifies the module that reported the fault, such as the ECM (Engine Control Module).

To interpret the code, connect a diagnostic tool to retrieve it. Look up the SPN and FMI in a chart or manufacturer guide to determine the exact issue. 

For example, SPN 102 FMI 3 CM 0 points to a high voltage problem in the intake manifold pressure system. Knowing how to break down these codes makes diagnosing and fixing issues much more straightforward.  

List of permanent DTC codes

How to Read and Interpret DTC Codes

Reading and understanding DTC codes is simple with the right tools. A code reader or scan tool is needed to detect and display the codes. These tools plug into your vehicle and show a five-character code that points to specific issues. Here’s how to do it:

Find the Diagnostic Port

Look for the 16-pin OBD-II port, usually under the dashboard on the left side of the steering wheel. Check the vehicle’s manual for the exact location if you can’t find it.

Connect the Scanner

Plug the scanner into the diagnostic port. Turn on the ignition or start the engine so the scanner can connect to the vehicle’s system.

Access the Codes

Use the menu on the scan tool to choose the “Read Codes” option. The scanner will then show the codes related to any faults it finds.

Understand the Codes

If your scanner has this feature, write down the codes or transfer them to a computer or phone using Wi-Fi, USB, or Bluetooth. Use the vehicle’s manual, the manufacturer’s website, or a DTC database to look up the meaning of the codes. 

Advantages of DTC Fleet Telematics Solutions

DTC Fleet Telematics Solutions, like Matrack, help fleet managers monitor vehicle health and address issues quickly by connecting to the vehicle’s diagnostic port.

Real-Time Fault Code Access

Managers receive fault codes immediately when a problem arises, making identifying and fixing issues quickly easy.

Proactive Issue Resolution

Early alerts notify managers about potential problems, reducing the chances of breakdowns and keeping vehicles on the road.

Improved Maintenance Awareness

The system highlights needed repairs and servicing, helping catch minor issues before they become expensive problems.

Enhanced Fleet Performance

Continuous monitoring ensures vehicles operate efficiently, improving reliability across the fleet.

Timely Scheduled Maintenance

The system tracks service schedules, ensuring vehicles are maintained on time, extending their lifespan and reducing repair costs.

Reduced Driver Dependency

Faults are detected automatically, so managers don’t rely on drivers to report issues. This improves accuracy and speeds up repairs.

Efficient Vehicle Fleet Management

With detailed vehicle insights, managers can make better decisions, reduce costs, and keep the fleet running smoothly. Know more about vehicle fleet management. 

Frequently Asked Questions 

What does 0 DTC Code mean, and how can it be fixed?

0 DTC is a generic error code that points to common vehicle issues. To fix it, connect a scanner to find the code and check its meaning in an OBD-II code list. Once you know the issue, fix the problem, like replacing a part or repairing a fault.

What Does DTC Severity Mean?

DTC severity shows how serious a vehicle issue is and how quickly it needs to be fixed. High-severity codes mean critical problems, like engine issues, that need immediate attention. Low-severity codes are minor problems, like a loose gas cap, that can be fixed later.

How long does it take for permanent DTC codes to be erased?

Permanent DTC codes clear after the problem is fixed and the vehicle completes several drive cycles. A drive cycle includes starting the engine, driving, and stopping. If the code isn’t clear, you may need to use a diagnostic tool.  

Conclusion

DTC codes help you quickly find and fix problems in your vehicles. They show clear details about what’s wrong and where to look.

Fleet telematics systems make it even easier by giving real-time alerts and maintenance updates. This keeps vehicles running smoothly, saves money, and avoids delays.