A Guide to Understanding DTC Codes

What is a DTC Code? 

A DTC, short for Diagnostic Trouble Code, is a code used to diagnose malfunctions in a vehicle or heavy equipment. While the malfunction indicator lamp (MIL)—also known as the check engine light—simply alerts drivers that there is an issue, a DTC identifies what and where the issue is. DTCs are also called engine vehicle fault codes codes, and can be read with a scanner that plugs directly into the port of a vehicle.

Having a basic understanding of DTCs can be helpful for drivers and fleet managers. Read on to learn more about DTCs, how they work, and how you can read them.

Where do DTCs come from?

DTCs are generated by the vehicle’s on-board diagnostics (OBD) system whenever a fault is detected. The OBD both diagnoses the fault and displays the DTC through visible warnings such as the illumination of a check engine light. It is also what allows external devices, such as an OBD scanner, to interact with a vehicle’s onboard computer system.

 Over the years, several different versions of OBD interfaces were used—these interfaces, now classified as OBD-I, largely varied from manufacturer to manufacturer. Today, there are two main standards that people can use to read DTCs.

• OBD2 -- best for light- and medium-duty vehicles

• J1939 -- best for heavy-duty vehicles and heavy equipment

With the implementation of OBD-II, a standard DTC list  that contains codes that are common to all manufacturers was created by the Society of Automotive Engineers (SAE).

Though OBD-II is widely used, it is important to note that manufacturer-specific DTCs still exist. Manufacturers may create their own DTCs to supplement the list of universal codes if the vehicle requires it, but more often than not these manufacturer-specific codes are difficult to interpret without a mechanic.

Is your vehicle OBD-II or J1939 compliant?

OBD-II compliant: 

• Cars and light trucks built and sold in the United States after January 1, 1996 were required to be OBD-II equipped.

J1939 compliant: 

• Most diesel engine manufactured vehicles or equipment are J1939 compliant 

• The industry standard for most heavy-duty trucks and buses

If you are unsure whether your vehicle is OBD-II or J1939 compliant, check your vehicle manual, research online, or consult with your dealership to confirm compatibility.

How do you read an OBD-II DTC?

DTC codes are five characters long. Each character in the DTC provides a different piece of information about the vehicle’s problem. 

The first character is always a letter. It indicates which control system has an issue, and has the following possible values and meanings:

• P (powertrain) refers to the engine, transmission, fuel system, and associated accessories. 

• C (chassis) refers to mechanical systems generally outside the passenger compartment such as steering, suspension, and braking.

• B (body) refers to parts mainly found in the passenger compartment area. 

• U (network) refers to the vehicle’s onboard computers and related systems.

The second character is a digit, typically 0 or 1, and shows whether or not the code is standardized. 

• 0 indicates that the code is generic, standardized SAE (Society of Automotive Engineers) code. Generic codes are adopted by all cars that follow the OBD-II standard.

• 1 indicates that the code is vehicle manufacturer-specific.These codes are unique to a specific car make or model and are typically less common. 

• 2 or 3 are more rare and their meanings are dependent on the preceding letter of the code. Most of the time, 2 or 3 indicates that a code is manufacturer-specific, with only a few exceptions. 

The third character is also a digit, ranging from 1 to 8. This reveals the subsystem at fault.

• 1 refers to the fuel or air metering system

• 2 refers to the fuel or air metering injection system

• 3 refers to the ignition system

• 4 refers to the emissions system

• 5 refers to the vehicle speed controls and idle control system

• 6 refers to the computer output circuit

• 7 and 8 indicate that the issue is transmission-related

The fourth and fifth characters are read together as a two-digit number between 0 and 99 known as the specific fault index. These characters identify the exact issue of the vehicle. 

If you are unsure about what your code means, check with your vehicle manufacturer or your vehicle manual for explanations.

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How do you interpret a DTC?

Each DTC corresponds to a specific vehicle malfunction, such as a misfire in engine one. The following common DTC, P0128, would be read like this: 

• The first letter is P, which means that the issue lies in the powertrain.

• The second character is 0, indicating that the following code is not manufacturer-specific code. It is standardized and can be interpreted according to SAE definitions.

• The third character is 1, which reveals that the issue is specifically within the fuel and air metering subsystem.

• The last two digits make up the specific fault index 28. 

Based on this reading, it’s clear that code P0128 refers to an issue of the engine coolant temperature that’s below thermostat regulating temperature.

What are common DTCs?

Certain DTCs are more likely to appear than others. Below is a list of common DTCs you may encounter and their corresponding malfunctions.

• P0442: A small system leak in a vehicle’s evaporative emission control system. 

• P0606: A  malfunction in the power train control module (PCM), also known as the engine control module (ECM). 

• P0101:  A fault in the mass air flow (MAF) sensor or circuit.

• P0110: A malfunction in the intake air temperature sensor circuit.

• P0500:  A vehicle speed sensor malfunction.

• P0706: Fault in the transmission range sensor circuit range.

It is important to note that while DTC’s are useful in identifying a vehicle’s malfunction, they are not a way to decipher why the malfunction occurred. A vehicle needs to be further diagnosed by a mechanic to identify and address the cause of the issue.

How do you read a J1939 DTC?

A J1939 DTC contains four fields that relay information about the fault being reported by DTC. These fields are:

1. Suspect Parameter Number (SPN): Represents the SPN with error. Every defined SPN can be used in a DTC.

2. Failure Mode Identifier (FMI): Represents the nature and type of error that occurred, e.g., value range violation (high or low), sensor short-circuits, incorrect update rate, calibration error.

3. Occurrence Counter (OC): A counter that counts the occurrence of the error condition for each SPN and stores this even when the error is no longer active.

4. SPN Conversion Method (CM): Defines the byte alignment within the DTC. The value “0” represents the method shown in graphic “Structure of a DTC”. If CM has the value “1”, a distinction must be made between three previously valid methods; this must be known for the system.

DTC scan tools for fleet managers 

Scanners that read DTC, known as OBD-II scanners, are widely available for purchase. But if you’re a fleet manager responsible for numerous vehicles, having to manually check a vehicle every time the check engine light turns on can be inefficient. 

That’s why it’s important to select a fleet management solution, like Samsara, that includes maintenance tools that make identifying a DTC a fast and easy process. Samsara vehicle gateways plug in directly to the vehicle’s OBD-II port and send vehicle-related data—including DTCs—to the cloud. This means if a DTC occurs, it will automatically populate in your Samsara Dashboard or you can choose to be notified via an alert.

In the event that several of your fleet vehicles experience DTC issues, a telematics solution that includes a DTC tool can help you quickly identify what the issue is so you can quickly take appropriate steps to schedule maintenance as needed. 

Understand DTCs easily with telematics

Seeing an illuminated check engine light without knowing the reason behind it can be troublesome. That’s why it’s important to find a telematics solution, like Samsara, that helps you understand DTC so you can identify your vehicle’s issue and take action accordingly.

The Samsara vehicle gateway plugs directly into the OBD-II or the J1939 port of a vehicle and can thus relay any information regarding any DTC directly to the Samsara Dashboard. Furthermore, alerts can be triggered in the event that a DTC occurs by using the "Vehicle Fault" alert. You can also use Samsara’s DTC filtering feature to get alerts for the DTCs that matter most to your fleet.

For more information about how Samsara can help you read and address DTCs, sign up for a free trial or learn more about Samsara’s fleet maintenance solutions.