Diagnostic trouble codes (DTCs) are used to help pinpoint any issue in a vehicle’s computerized systems.

In this guide we will cover everything from detecting DTCs to finding effective fixes for those issues so that you can ensure maximum uptime for your vehicles and keep things running smoothly.

Understanding DTC Codes

Truck dtc codes

DTCs are a crucial part of ensuring the safe operation of your vehicle.

When a DTC is triggered, it will cause a warning light to appear on your dashboard. While typically it requires a mechanic or OBD-II scanner to retrieve the codes, understanding how DTCs work can help you diagnose and fix issues on your own.

With onboard diagnostic reading capabilities, your fleet management system can help you understand the specific truck DTC codes causing the malfunction. This means that while a driver may only see a warning light, your integrated fleet management software can provide you with the necessary information to address the issue.

It’s crucial to stay informed and take necessary action to keep your vehicle running safely and smoothly.

How do DTC Codes Work?


Your fleet’s vehicle diagnostic code falls into a standardized system that any mechanic or technician can interpret with ease. Once a problem is detected in the onboard diagnostic system, a corresponding DTC code is triggered, displaying as warning lights or codes on the dashboard for the driver to see.


The OBD-II (On-Board Diagnostics II) is a vehicle’s built-in self-diagnostic system. It has sensors that collect information from various vehicle parts to help mechanics pinpoint specific vehicle operation issues.

The primary purpose of this system is to identify and report any vehicle malfunction, from the engine to emission control systems.

When the onboard diagnostic system detects a problem, it triggers the corresponding DTC, which shows as a warning light or code on the dashboard for the driver. The DTC will then provide more context about the issue, allowing mechanics to identify and troubleshoot the error quickly.

To access the information stored in the OBD-II system, a mechanic needs to connect a scanner to the OBD-II port. This allows technicians to see a detailed view of the problem, allowing them to determine the necessary repair or maintenance accurately.

Critical and Non-Critical Codes

DTCs are classified as critical or non-critical based on the severity of the problem:

  • Critical codes indicate immediate action is necessary. These are urgent and significant malfunctions that can cause severe damage to the vehicle or harm the driver or passengers. Most codes that start with “P” or “C” (see below) are identified as critical systems, such as engine overheating, brake failure, or fuel leaks.
  • Non-critical codes are less severe issues that do not directly threaten the vehicle’s safety and operation. However, you still need to have your vehicle diagnosed or repaired to ensure optimal performance and functionality. Examples of non-critical codes refer to emissions issues or internal systems like the radio or A/C unit that offer comfort.

As part of your fleet safety management program, you must train your drivers never to ignore warning lights.

Although fleet managers may access real-time diagnostic information through a DTC scanner, drivers must promptly report any warning lights to the fleet manager or technician for safety and vehicle maintenance.

How to Read and Interpret DTC Codes

DTC codes are five characters long, starting with a letter that indicates the overall problem area of the malfunction:

DTC Formats

  1. Each code starts with a letter that indicates the overall problem area of the malfunction:
    • P – Powertrain, which includes the engine, transmission, and fuel systems
    • C – Chassis, which includes steering, suspension, and brakes
    • B – Body, which includes systems inside the car, such as A/C
    • U – Network, which includes all onboard computer systems
  2. The second letter of the DTC code indicates whether or not the code is standard or manufacturer-specific.
    • 0 indicates a standard code across all vehicles
    • 1, 2, or 3 indicates a manufacturer-specific code, with 2 and 3 relatively rare occurrences
  3. The third letter determines the subsystem:
    • 1 or 2 indicates an issue with fuel or air metering
    • 3 indicates an issue with the ignition
    • 4 indicates an issue with the emissions system
    • 5 indicates an issue with vehicle control or speed
    • 6 indicates an issue with computer circuitry
    • 7 and 8 indicate issues with the transmission
  4. The fourth and fifth letters in the code are read together as two-digit numbers. So if you see a code that says P0246, you would read it as P-0-2-46. This allows you to pinpoint the specific problem in your vehicle, indicating the system and subsystem to inspect and identify the root cause.

Let’s put it all together with an example. Say you receive a notification on your fleet management system that one of your vehicles triggered the DTC code P0300:

  • The letter P immediately tells you something is wrong with the engine, transmission, or fuel.
  • 0 means it’s a standard DTC code
  • 3 identifies the ignition as the issue
  • 00 refers to the cylinders.

Hence, the code P0300 signifies the detection of random or multiple-cylinder misfires (Random or Multiple Cylinder Fires Detected). This code is commonly accompanied by additional codes ranging from P0301 to P0308, explicitly identifying the cylinders experiencing the misfires.

Here’s a standard ECU fault code list:

P0420 or P0430Catalytic converter
P0141Oxygen sensor
P0445 or P0446EVAP system
P0171 or P0174Fuel consumption
C0221ABS/Wheel speed sensor

How to interpret J1939 DTCs

The codes will differ slightly if you own heavy-duty commercial vehicles or equipment. J1939 codes are four characters long, with each character representing a system that, when taken together, points you toward the specific problem for your vehicle.

This includes:

  • Suspect Parameter Number (SPN) represents the overall system affected by the malfunction, corresponding to a standard set of numbers.
  • Failure Mode Identifier (FMI) represents the nature and type of the error.
  • Occurrence Counter adds up the number of times this error occurs for every SPN.
  • SPN Conversion Method (CM) represents the byte alignment for the DTC code.

Final Thoughts

Knowing how to identify and interpret diagnostic trouble codes (DTCs) correctly is essential to prevent delays in repair time or additional costs due to misdiagnosis.

With the knowledge obtained in this blog post and part 2, you will be able to troubleshoot vehicle issues more accurately and longer uptimes for your vehicles.

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