Seeing the ‘check engine light’ illuminate on your dashboard without understanding why can be unsettling. That’s why having a way to decipher Diagnostic Trouble Codes (DTCs) is essential. Understanding these codes helps you pinpoint vehicle issues and take appropriate action.
What is a DTC?
DTC, which stands for Diagnostic Trouble Code, is a fault code used to identify malfunctions in a vehicle or heavy equipment. While the malfunction indicator lamp (MIL), commonly known as the check engine light, simply alerts drivers to a problem, a DTC specifies the nature of the issue and its location. DTCs are also referred to as vehicle engine fault codes and can be accessed using a scanner that connects directly to a vehicle’s port.
A basic understanding of DTCs can be valuable for both drivers and fleet managers. Continue reading to learn more about DTCs, how they function, and how to interpret them.
Where Do DTCs Come From?
DTCs are generated by the vehicle’s onboard diagnostic system (OBD) whenever a fault is detected. The OBD system diagnoses the fault and signals the DTC through visible warnings, such as turning on the check engine light. It also enables external devices, like an OBD scanner, to interact with the vehicle’s onboard computer system.
Over the years, various versions of OBD interfaces were used. These interfaces, now categorized as OBDI, varied significantly between manufacturers. Today, two primary standards are used to read DTCs:
- OBD2: Best suited for light- and medium-duty vehicles.
- J1939: Best suited for heavy-duty vehicles and equipment.
With the implementation of OBDII, the Society of Automotive Engineers (SAE) established a standardized DTC list containing fault codes common across all manufacturers.
Although OBDII is widely adopted, it’s important to note that manufacturer-specific DTCs still exist. Manufacturers can create their own DTC codes to supplement the universal code list if a vehicle requires it. However, these manufacturer-specific codes are often challenging to interpret without specialized mechanic knowledge.
Does Your Vehicle Use OBDII or J1939?
OBDII Compliant:
- Cars and light trucks manufactured and sold in North America after January 1, 1996, are required to be equipped with OBD-II.
J1939 Compliant:
- Most vehicles or equipment manufactured with diesel engines are J1939 compliant.
- It is the industry standard for most heavy-duty trucks and buses.
If you are unsure whether your vehicle is OBDII or J1939 compliant, consult your vehicle’s manual, research online, or check with your dealership to confirm compatibility.
How to Read an OBDII DTC Code
DTC codes consist of five characters. Each character within the DTC provides different information about the vehicle’s problem.
The first character is always a letter, indicating which control system has an issue. The possible values and their meanings are:
- P (Powertrain): Refers to the engine, transmission, fuel system, and associated accessories.
- C (Chassis): Relates to mechanical systems generally outside the passenger compartment, such as steering, suspension, and braking.
- B (Body): Indicates parts found primarily within the passenger compartment area.
- U (Network): Refers to the vehicle’s onboard computer and related systems.
The second character is a digit, typically 0 or 1, showing whether the code is standardized:
- 0: Indicates a generic, standardized SAE (Society of Automotive Engineers) code. All vehicles adhering to the OBDII standard adopt generic codes.
- 1: Indicates a manufacturer-specific code. These codes are unique to a particular car brand or model and are generally less common.
- 2 or 3: Rarer and their meanings depend on the preceding letter in the code. Mostly, 2 or 3 indicate a manufacturer-specific code, with 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 auxiliary emission controls system.
- 5: Refers to vehicle speed controls and idle control system.
- 6: Refers to the computer output circuit.
- 7 and 8: Indicate a transmission-related problem.
The fourth and fifth characters are read together as a two-digit number between 0 and 99, known as the specific fault index. These digits identify the exact vehicle problem.
If you are uncertain about your code’s meaning, consult your vehicle manufacturer or your manual for further explanation.
How to Interpret a DTC
Each DTC corresponds to a specific vehicle malfunction, such as an engine misfire in cylinder one. Here’s an example of how a common DTC, P0128, would be interpreted:
- The first letter is P, meaning the problem lies within the Powertrain.
- The second character is 0, indicating this is not a manufacturer-specific code. It is standardized and can be interpreted according to SAE definitions.
- The third character is 1, revealing the issue is specifically within the fuel and air metering subsystem.
- The last two digits form the specific fault index 28.
Based on this breakdown, it’s clear that code P0128 refers to an issue with the engine coolant temperature being below the thermostat regulating temperature.
Alt: OBD2 DTC code P0128 breakdown, showing each digit’s meaning for powertrain, generic code, fuel/air metering, and specific fault index.
Common DTC Codes to Be Aware Of
Certain DTC codes are more prevalent than others. Here is a list of common DTCs you might encounter and their corresponding malfunctions:
- P0442: Small leak detected in the vehicle’s evaporative emission control system.
- P0606: Malfunction in the powertrain control module (PCM), also known as the engine control module (ECM).
- P0101: Mass Air Flow (MAF) sensor or circuit range/performance problem.
- P0110: Intake Air Temperature Sensor Circuit Malfunction.
- P0500: Vehicle Speed Sensor Malfunction.
- P0706: Transmission Range Sensor Circuit Range/Performance.
It’s crucial to remember that while DTCs are helpful for identifying vehicle malfunctions, they don’t explain why the problem occurred. A vehicle should be further diagnosed by a mechanic to pinpoint and address the root cause.
How to Read a J1939 DTC Code
A J1939 DTC contains four fields that transmit information about the reported fault. These are:
- Suspect Parameter Number (SPN): Represents the specific SPN that is in error. Each defined SPN can be used in a DTC.
- Failure Mode Identifier (FMI): Represents the nature and type of error that occurred, for example, value range violation (high or low), sensor shorts, incorrect update rate, calibration error.
- Occurrence Count (OC): A counter that tracks the occurrence of the error condition for each SPN and stores it even when the error is no longer active.
- SPN Conversion Method (CM): Defines the byte alignment within the DTC. A value of “0” represents the method shown in the “Structure of a DTC” graphic. If CM has a value of “1”, a distinction must be made between three previously valid methods; this should be known by the system.
Alt: Diagram illustrating the structure of a J1939 DTC code, highlighting SPN, FMI, OC, and CM fields and their data byte allocation.
DTC Scanning Tools for Fleet Managers
Scanners that read DTCs, known as OBDII scanners, are widely available for purchase. However, if you’re a fleet manager responsible for numerous vehicles, manually checking each vehicle every time the check engine light comes on can be inefficient.
This is why selecting a fleet management solution, like Samsara, that includes maintenance tools to streamline DTC identification is important. Samsara vehicle gateways plug directly into the vehicle’s OBDII port and transmit related data to the cloud, including DTCs. This means if a DTC occurs, it will automatically appear on your Samsara dashboard, or you can choose to receive a notification via an alert.
If multiple vehicles in your fleet experience DTC issues, a telematics solution with a DTC tool can help you quickly identify the problem so you can take the necessary steps to schedule maintenance.
Easily Understand DTCs with Telematics
Seeing a check engine light illuminate on your dashboard without understanding the reason can cause stress. That’s why utilizing a telematics solution, such as Samsara, which helps you understand Diagnostic Trouble Codes (DTCs), is so valuable. It allows you to quickly identify vehicle problems and take appropriate actions.
Samsara’s vehicle gateway connects directly to a vehicle’s OBDII or J1939 port and can transmit information related to any DTCs directly to the Samsara dashboard. Furthermore, alerts can be activated when a DTC occurs using the “Vehicle Fault” alert feature. You can also utilize Samsara’s DTC filtering feature to receive alerts for the DTCs most critical to your fleet.
To learn more about how Samsara can help you read and address DTCs, sign up for a free trial or explore more about Samsara’s fleet maintenance solutions.
