What are DTC Codes? – How They Work & How To Identify Them

If you manage a fleet, staying on top of any engine or other vehicle issues is critical for the safety of your drivers. It also helps prevent more costly repairs.

This is where Diagnostic Trouble Codes (DTC) come in — codes that a vehicle’s onboard computer relays about any potential issues.

How do these codes work, and how do you interpret them? Most importantly, how can you gain remote access to them for your fleet?

Keep reading to learn more.

In this article, we’ll explain what DTC codes are and how to interpret them. We’ll also look at how you can manage the maintenance of your fleet with fleet telematics.

DTC meaning: What is a DTC code?

A DTC stands for Diagnostic Trouble Code, which 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 and can be read with a scanner that plugs directly into the port of a vehicle.

A DTC code is a series of diagnostic trouble codes used by a vehicle’s onboard diagnostics (OBD) system to alert you when a vehicle experiences a malfunction. Different codes have different DTC meanings and represent specific problems in your vehicle.

When the vehicle’s OBD system detects a problem, it generates a specific DTC code and transmits the alert to the vehicle’s instrument panel as a warning light. In vehicles equipped with a telematics system, the alert can be delivered directly to the fleet. The system can be set up to deliver the alert right to the maintenance department.

These codes were created by the Society of Automotive Engineers (SAE) to help vehicles comply with emission regulations. SAE is now called SAE International, and it is a professional organization that develops standards for automotive engineers.

How diagnostic trouble codes (DTCs) work

Whenever your car experiences a problem or detects a fault, a DTC code is generated by your vehicle’s OBD system.

Once detected, the OBD system will alert you by displaying the DTC code via visible warnings like an illuminated check engine light (Malfunction Indicator Lamp).

A DTC code lets external devices, such as an onboard diagnostics scanner (code reader), interact with your car’s OBD system.

Initially, there were several versions of onboard diagnostics interfaces that largely varied depending on the vehicle manufacturer.

But today, there are two main standards used to read DTC codes:

  • OBD-II: For light and medium-duty vehicles
  • J1939: For heavy-duty vehicles and equipment

With the implementation of OBD-II, the Society of Automotive Engineers (SAE) created a standard DTC list full of common codes for every vehicle manufacturer. These codes help cars stick to emission regulations and save the environment.

Note: OBD-II may be used the most, but DTC codes specific to certain manufacturers do exist. A vehicle manufacturer may create their own DTC codes to add to the universal code list if certain vehicle models need it.

Are all DTCs critical?

There are several reasons that your vehicle’s check engine light can be illuminated, but not all of them are equally important. The critical nature of a code is driven by what is affected in the malfunction. DTC codes can fall into two categories: critical and non-critical codes.

Critical DTC codes need urgent attention because they can cause immediate and severe damage. A good example of this could be a high engine temp. non-critical codes aren’t urgent, but it’s crucial that DTC codes are correctly diagnosed to be sure.

How Do You Interpret OBD-II DTC Codes?

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

Interpret OBD-II DTC Codes

Codes are standard, and you’ll know what area of your vehicle the code refers to if you understand the structure of the code and the standard abbreviations.

Here’s an overview of what each one means.

First DTC character

The first DTC character is always a letter. There are four types of codes:

  • P codes: “P” indicates a problem with the powertrain. It includes the engine, transmission, drivetrain, and fuel system
  • C codes: “C” indicates a problem with the chassis. It refers to mechanical systems outside the passenger compartment, such as steering, suspension, and braking.
  • B codes: “B” indicates a problem with the body. It covers parts that are found in the passenger compartment area.
  • U codes: “U” indicates a problem with the vehicle’s onboard computers and integration functions that the OBD manages.

Second DTC character

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 rarer 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.

Third DTC character

If the second DTC character is a “0,” then the third character helps you determine which subsystems are malfunctioning. There are eight numbers:

  • 0 – Fuel and air metering and 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 control, idle control systems, and auxiliary inputs
  • 6 – Computer and output circuit
  • 7 – Transmission

You may also see an A, B, or C, which can refer to hybrid propulsion systems.

Fourth and fifth DTC character

The fourth and fifth numbers from the vehicle’s onboard diagnostics system will tell you the specific fault index number to pinpoint the underlying issue. These numbers can range between 0 to 99.

Examples: Here’s an example of a complete code: P0782

  • P Means – Powertrain,
  • 0 Means – Generic,
  • 7 Means – Transmission,
  • 82 Means – 2-3 shift malfunction.

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:

  • Suspect Parameter Number (SPN): Represents the SPN with an error. Every defined SPN can be used in a DTC.
  • 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, and calibration error.
  • 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.
  • SPN Conversion Method (CM): Defines the byte alignment within the DTC. The value “0” represents the method shown in the 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.

How To Read Diagnostic Trouble Codes (Dtcs)

Reading DTCs requires a scanner. This tool’s 16-point trapezoidal connector plugs into the vehicle and interprets the OBD code. Accessing these codes with a scanner can be done by following these steps.

Step 1: Plug into the OBD port.

OBDII-equipped vehicles have a standardized 16-pin diagnostic link connector (DLC) used to access the vehicle’s onboard diagnostics. Each vehicle has a different location for the OBD port, but most often, it’s found under the driver-side dashboard. The owner’s manual should provide the precise location.

Step 2: Turn on the ignition.

While you don’t have to actually turn the engine over, switching on the ignition will power the scanner. If the scanner doesn’t come on when you turn on the ignition, re-check the connection between the scanner and the connector pins. If the plug is connected snugly, plug something into the vehicle’s auxiliary power outlet to see if it works—the ODB-II scanner runs on the same circuit.

Step 3: Enter the necessary information.

Since DTCs can be manufacturer-specific, some scanners require additional information. This could include the vehicle identification number (VIN), model, make, or engine type.

Step 4: Navigate the OBD codes.

Your scanner may alert you to active codes and pending codes. Active codes refer to current malfunctions that have tripped your check engine light. Pending codes refer to intermittent faults that the system needs to see happen in consecutive warm-up cycles to be considered active. If the fault doesn’t reappear in 40 warm-up cycles, the code gets cleared.

Step 5: Interpreting the DTC

Once you have the code, you can discern what it means. The information listed above will help you hone in on the nature of the problem, but to get the exact details, you’ll probably need to consult a website or use an app that interprets the thousands of OBD codes and their fixes.

How Do You Clear A DTC Code?

Most first-generation OBDs in vehicles used before OBD-II allowed you to disconnect a battery cable or the computer’s power source, and it would erase the fault codes.

The voltage loss would erase the computer’s temporary memory, and the Check Engine Light would stop lighting up. However, the light would turn back on, and the codes would reset when the original issue recurred.

With the latest OBD computer systems, DTCs get stored in a “non-volatile” memory that remains even if the battery or computer is disconnected. The fault codes are kept intact until you or the mechanics clear them using a scan tool.

Clearing DTCs by disconnecting the computer’s power supply or battery isn’t recommended since it can lead to a climate control system and electronic radio preset loss.

It can also result in losing the engine computer system’s “learned” memory which is almost impossible to recover since it contains the adjustments made over time to compensate for driving habits and engine wear.

The engine’s computer might also need to be put through a particular learning process to relearn the proper transmission operation in case of power loss on vehicles where the engine computer also controls the electronic transmission.

Popular DTC codes list

While vehicle trouble can vary depending on the vehicle’s condition or issues it encounters, there are some DTCs and their corresponding malfunctions that most fleets encounter, such as:

  • P0101: A fault in the mass air flow (MAF) sensor or circuit.
  • P0110: A malfunction in the intake air temperature sensor circuit.
  • P0442: Small system leak within the car’s evaporative emission control system.
  • P0300: Random or multiple cylinders misfire detected.
  • P0215: Engine shutoff solenoid malfunction.
  • P0500: A malfunction in the vehicle’s speed sensor
  • P0606: A PCM (or ECM) powertrain malfunction
  • P0706: A transmission range sensor circuit range fault
  • P0650: Malfunction Indicator Lamp (check engine light) control circuit malfunction
  • P1108: Dual alternator battery lamp circuit malfunction
  • P1794: Battery voltage circuit malfunction
  • B1927: Faulty passenger side airbag
  • B1203: Fuel sender circuit short to battery
  • P0352: Ignition coil B primary or secondary circuit malfunction
  • P0353: Ignition coil C primary or secondary circuit malfunction
  • P0130: O2 sensor (oxygen sensor) circuit malfunction (Bank 1, Sensor 1)
  • P0141: O2 sensor (oxygen sensor) heater circuit malfunction (Bank 1, Sensor 2)
  • P0654: Engine RPM (revolution/rotations per minute) output circuit malfunction
  • P0120: Throttle pedal position sensor or switch A circuit malfunction
  • P0656: Fuel level output circuit malfunction

DTCs provide useful information to help identify vehicle malfunctions, but they are not the ultimate way to figure out why the malfunction happened.

With the help of DTCs, malfunctioning vehicle components will need further assessment and diagnosis by mechanics to pinpoint and fix the cause of the issue (or issues).