What Is A DTC, Its Types and How To Read It

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In the dynamic field of automotive diagnostics, it’s important to know what is a DTC. Diagnostic Trouble Codes (DTCs) are vital indicators of potential issues within a vehicle. These codes are generated by the onboard diagnostics system and provide valuable insights into the health and performance of the vehicle.

What Is A DTC ?

In this article, we’ll understand DTCs, explore their types, meanings, and how to interpret them effectively. We’ll decode the mysteries behind these alphanumeric messages and learn their effective navigation.

What Are DTC Codes?

DTC codes, or Diagnostic Trouble Codes, are created by the Society of Automotive Engineers (SAE). These codes serve as a vehicle’s self-diagnostic system. The codes are alphanumeric and generated by the vehicle’s on board diagnostics system (OBD) when it detects any abnormalities or malfunctions in the vehicle.

A DTC is composed of a combination of letters and numbers. It provides specific information about the nature and location of the problem. These codes are essential for both vehicle owners and automotive technicians, as they offer valuable insights into the vehicle’s health. When a problem arises, the vehicle’s check engine light illuminates, signaling the presence of a DTC.

DTC codes are divided into two main categories: 

  • Generic: Generic codes are standardized across all vehicle makes and models, making them universally applicable.
    • OBD-II: Used in light (6,000 to 10,000 lbs) and medium-duty vehicles (10,001 to 26,000lbs).
    • J1939: Used in heavy-duty vehicles (26,001 to over 33,000 lbs) like city transit buses, refuse trucks, and cement trucks.
  • Manufacturer-specific: Manufacturer-specific codes are unique to particular brands or models. These codes offer more detailed information tailored to their systems.

Retrieving DTC codes typically requires an OBD scanner or diagnostic tool. Once retrieved, the codes can be interpreted using a code reader’s manual, online databases, or manufacturer-specific resources.

DTC codes play a vital role in automotive diagnostics — providing valuable information about a vehicle’s health and performance. By understanding and interpreting these codes accurately, you can address issues promptly, ensuring optimal vehicle function and safety.

What Drivers Need To Know About DTCs

As a driver, understanding DTC codes is crucial. It helps identify issues early on, allowing for prompt diagnosis and repair. More on that:

  • DTCs reveal various issues like engine misfires, sensor malfunctions, or emissions problems, impacting vehicle performance and safety.
  • Drivers can retrieve DTCs using OBD scanners or diagnostic tools that plug into the vehicle’s OBD port.
  • Once retrieved, drivers can interpret DTCs with the scanner’s manual, online resources, or manufacturer-specific databases.
  • Addressing DTCs promptly is crucial to prevent further damage to the vehicle and ensure optimal performance. For complex DTCs or issues beyond DIY troubleshooting, drivers should seek help from certified mechanics only.
  • Ignoring DTCs can pose safety risks, especially if they relate to critical systems like brakes or airbags. It may also lead to costly repairs later on, making awareness and a proactive approach economically wise.
  • Drivers can educate themselves about DTCs through online resources, manuals, and automotive courses to better understand their vehicles.
  • In regions with emissions regulations, ignoring DTCs can have legal consequences, leading to fines or even vehicle impoundment.

Where Do DTCs Come From?

DTCs originate from a vehicle’s Onboard Diagnostics System (OBD) — a computerized system that monitors the various components and systems within a vehicle. OBD systems have evolved over time, with newer generations offering enhanced diagnostic capabilities and more comprehensive monitoring of vehicle systems. The OBD system monitors sensors and components throughout the vehicle, such as those in the engine, transmission, and emissions control system. 

When anomalies or malfunctions occur within these components or systems, the OBD system generates a DTC to indicate the problem. Various factors can trigger the generation of DTCs, including sensor readings falling outside predetermined thresholds, system voltage irregularities, or mechanical failures. 

The presence of a DTC typically triggers the vehicle’s check engine light or malfunction indicator lamp (MIL) to alert the driver to the issue. Some OBD systems use diagnostic trouble trees, flowcharts, or decision trees that guide technicians through the diagnostic process based on specific DTCs. DTCs are recorded and stored in the vehicle’s onboard computer memory, allowing technicians to retrieve and interpret them using diagnostic tools. 

Retrieval methods for DTCs include OBD scanners, diagnostic tools, or specialized software that interfaces with the vehicle’s OBD system. The communication protocols used to retrieve DTCs vary depending on the vehicle’s make, model, and model year. Newer vehicles often use standardized protocols such as OBD-II. 

Once retrieved, DTCs help technicians diagnose the underlying issue and perform necessary repairs to address the problem and clear the code.

Types of DTC Codes

We’ve already discussed generic and manufacturer-specific codes. Further, DTC codes can be bifurcated into two even more important categories:

Critical Codes

Critical codes represent urgent DTC codes that can cause severe vehicle damage. For instance, high engine temperatures or low coolant levels may signal impending engine failure.

The ability to assess these critical codes with real-time vehicle diagnostics helps fleet managers and technicians evaluate the severity of the issue, offer immediate advice, and locate the nearest service center for timely resolution. This, in turn, helps avoid costly emergencies, fines and even potential impoundment of the vehicle.

Non-Critical Codes

As the name suggests, non-critical codes don’t require immediate action. But they still need attention to maintain environmental standards and vehicle performance. 

For instance, these codes highlight emission-related engine problems, excessive pollution, etc. Specifically, non-critical codes involve the Malfunction Indicator Lamp (MIL), commonly called the Check Engine Light. 

Frequently Encountered DTC Codes Directory

  • P0101 – A Mass Air Flow (MAF) circuit or sensor fault
  • P0110 – An intake air temperature sensor circuit malfunction
  • P0442 – A small system leak within the vehicle’s evaporative emission control system
  • P0500 – A malfunction in the vehicle’s speed sensor
  • P0606 – A PCM (or ECM) powertrain malfunction
  • P0706 – A transmission range sensor circuit range fault

While the codes do come handy, it’s important to remember that these codes do not decode the cause of the malfunction. Usually, the malfunctioning components will require expert diagnosis to understand the cause and fix the issue.

Steps To Clear DTC Fault Codes

Here’s a step-by-step guide on how to clear DTCs for vehicle maintenance and troubleshooting:

Steps To Clear DTC Fault Codes
  1. Locate and Plug In the OBD Port: You’ll typically find the OBD port under the dashboard on the driver’s side, near the steering column. If not, the owner’s manual will help you with the exact location. Plug in your OBD scanner or diagnostic tool into this port.
  1. Power On the Scanner: Turn on the OBD scanner or diagnostic tool. You may also need to enter the vehicle’s VIN or select the appropriate vehicle make and model for some scanners.
  1. Retrieve DTCs: Use the scanner’s “Read Codes” or similar option to retrieve the DTCs from the vehicle’s onboard computer.
  1. Interpret DTCs: Review the alphanumeric DTCs displayed on the scanner’s screen — each code consists of different letters and numbers that provide specific information about the nature of the issue and its location.
  1. Refer to DTC Definitions: Consult the scanner’s manual, online resources, or manufacturer-specific databases to interpret the retrieved DTCs and understand the problem indicated by the codes.
  1. Record DTCs: Note the DTCs for future reference by saving or printing them on your scanner.
  1. Clear DTCs: Depending on the scanner’s capabilities and the issues detected, you may have the option to clear the DTCs from the vehicle’s onboard computer. Clear the DTC after resolving the problems.
  1. Address Underlying Issues: Diagnose and address the underlying issues affecting your vehicle. Seek professional assistance from a certified technician, based on the severity and complexity of the problems indicated by the DTCs.
  1. Perform Follow-Up Checks: Perform follow-up checks by checking the DTCs again to ensure successful problem resolution.

Deciphering OBD-II DTCs

We understand that over 5,000 OBD-II and manufacturer-specific codes are difficult to memorize. However, familiarity with commonly encountered codes can help troubleshoot issues quickly.

The check engine or malfunction indicator light (MIL) signals that the vehicle’s computer has logged a Diagnostic Trouble Code (DTC). This signal necessitates the use of a diagnostic tool or scanner for retrieval and analysis of the code.

Let’s understand how to decode these codes. Basically, a DTC comprises five characters and each character conveys specific information about the fault:

The first character denotes the location of the issue:

P = Powertrain

B = Body

C = Chassis

U = Network

The second character indicates whether it’s a generic OBD-II code or a manufacturer-specific code. A zero denotes a generic code.

The third character identifies the vehicle’s system at fault — ranging from fuel and air metering to hybrid propulsion faults.

The last two characters specify the particular fault to pinpoint the issue’s precise location and the necessary action.

For instance, consider “P0171” in the OBD-II codes list, it signifies that there’s too much air and not enough fuel for the engine to run efficiently.

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The bottom line with DTCs — they’re like your car’s way of talking to you, letting you know when something’s up under the hood. When that check engine light comes on, it’s trying to tell you something important. By understanding those DTC codes, you’re basically learning your car’s language.

With the rise of OBD-II systems, diagnosing car troubles has become way easier. But it’s not just about knowing the codes; it’s about knowing what to do with them. Clearing codes might seem like a quick fix, but it’s crucial to address the underlying issues first.

So, next time that check engine light pops up, grab your trusty bluetooth ELD by TruckX, read those codes like a pro on the fleet management system, and start fixing the issue. Don’t hesitate to reach out to a professional, it’s better to get things sorted properly than to risk more damage.

So, here’s to smooth rides and clear check engine lights! With a little know-how, you’ll be cruising down the road with confidence in no time.


How do I know if a DTC indicates a serious problem or a minor issue?

The severity of a DTC depends on various factors — specific code, the vehicle’s make and model, and the symptoms observed. Consult a mechanic or refer to vehicle-specific resources for guidance.

Can DTCs be triggered by temporary issues, or do they always indicate persistent problems?

Both, temporary and persistent issues can trigger DTCs. Some codes may clear automatically after a certain number of drive cycles if the problem doesn’t recur. Other issues may persist until resolution.

What should I do if I retrieve a DTC but don’t notice any symptoms in my vehicle?

It’s essential to investigate the DTC to prevent any potential problems from worsening over time. Ignoring DTCs can lead to increased fuel consumption, reduced performance, and even engine damage.

Are there any DIY methods for diagnosing and resolving DTCs without using a diagnostic tool?

While basic troubleshooting steps like checking for loose connections or damaged wires can sometimes help, diagnosing and resolving DTCs requires a diagnostic tool and, in some cases, specialized equipment or expertise.

Can aftermarket modifications or accessories trigger DTCs?

Yes, aftermarket modifications or accessories can sometimes interfere with the vehicle’s systems and trigger DTCs. It’s essential to ensure that any modifications are compatible with the vehicle and installed properly.

Is it possible for DTCs to be triggered by environmental factors?

Yes, extreme temperatures, altitude changes, and driving conditions might trigger DTCs, especially if they cause sensors or components to malfunction temporarily.

Can DTCs be prevented, or are they an inevitable part of vehicle ownership?

DTCs are an inevitable part of vehicle ownership. However, the complexity of modern vehicles, regular maintenance and timely repairs can minimize their occurrence and severity.

How can I stay informed about DTCs and their implications for my vehicle?

You can stay ahead of DTCs and their implications with vehicle owner’s manuals, online forums, and automotive websites. Additionally, regular maintenance and inspections by qualified mechanics can help identify and address DTCs effectively.