Seeing the ‘check engine’ light illuminate on your dashboard without knowing why can be unsettling. That’s why having a telematics solution that helps you decipher Diagnostic Trouble Codes (DTCs) is invaluable. Understanding these codes empowers you to identify your vehicle’s issues and take appropriate action.
What is a DTC Code?
A DTC, short for Diagnostic Trouble Code, is a fault code used to pinpoint malfunctions in a vehicle or heavy equipment. While the Malfunction Indicator Light (MIL), also known as the check engine light, merely alerts drivers to a problem, a DTC specifies the problem 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 diagnostic port.
A basic understanding of DTCs can be beneficial for both drivers and fleet managers. Read on to delve deeper into DTCs, how they function, and how to interpret them.
The Origin of DTCs
DTCs are generated by a vehicle’s On-Board Diagnostics (OBD) system 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 interface with the vehicle’s onboard computer system.
Over the years, various iterations of OBD interfaces were employed. These interfaces, now categorized as OBDI, varied significantly across manufacturers. Today, two primary standards are used to read DTCs:
- OBD2: Best suited for light- and medium-duty vehicles.
- J1939: Ideal for heavy-duty vehicles and equipment.
With the introduction of OBDII, the Society of Automotive Engineers (SAE) established a standardized list of DTCs. This list contains fault codes common across all manufacturers.
While OBDII is widely adopted, it’s crucial to note that manufacturer-specific DTCs still exist. Manufacturers can create proprietary DTCs to supplement the universal code list if required by the vehicle. However, these manufacturer-specific codes are often challenging to interpret without specialized mechanic knowledge.
OBDII vs. J1939: Which Does Your Vehicle Use?
OBDII Compliant:
- Cars and light trucks manufactured and sold in North America after January 1, 1996, were mandated to be equipped with OBD-II.
J1939 Compliant:
- Most vehicles or equipment powered by diesel engines are J1939 compliant.
- It is the industry standard for the majority of heavy-duty trucks and buses.
If you are uncertain whether your vehicle is OBDII or J1939 compliant, consult your vehicle’s manual, conduct online research, or check with your dealership for confirmation.
Decoding an OBDII DTC: A Step-by-Step Guide
OBDII DTC codes are composed of five characters. Each character within the DTC provides specific information about the vehicle’s problem.
The first character is always a letter, indicating the affected control system:
- P (Powertrain): Relates to the engine, transmission, fuel system, and associated accessories.
- C (Chassis): Concerns mechanical systems generally outside the passenger compartment, such as steering, suspension, and braking.
- B (Body): Refers to parts primarily located within the passenger compartment area.
- U (Network): Indicates issues with the vehicle’s onboard computer and related systems.
The second character is a digit, typically 0 or 1, signifying whether the code is standardized:
- 0: Indicates a generic SAE (Society of Automotive Engineers) standardized code. All vehicles adhering to the OBDII standard adopt generic codes.
- 1: Denotes a manufacturer-specific code. These codes are unique to a specific car brand or model and are generally less common.
- 2 or 3: Rarer and their meanings depend on the preceding letter in the code. Most often, 2 or 3 indicate a manufacturer-specific code, with only a few exceptions.
The third character is also a digit, ranging from 1 to 8, revealing the malfunctioning subsystem:
- 1: Refers to the fuel or air metering system.
- 2: Concerns the fuel or air metering injection system.
- 3: Relates to the ignition system.
- 4: Pertains to the emission control system.
- 5: Concerns vehicle speed controls and the idle control system.
- 6: Refers to the computer output circuit.
- 7 and 8: Indicate a transmission-related issue.
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 pinpoint the exact vehicle problem.
If you are unsure about the meaning of your code, consult your vehicle manufacturer or your manual for further clarification.
Interpreting a DTC: An Example
Each DTC corresponds to a specific vehicle malfunction, such as an engine misfire in cylinder one. Here’s how a common DTC, P0128, would be interpreted:
- P: The first letter ‘P’ signifies that the problem lies within the powertrain system.
- 0: The second character ‘0’ indicates that the code is not manufacturer-specific. It’s a standardized code interpretable according to SAE definitions.
- 1: The third character ‘1’ reveals that the problem is specifically within the fuel and air metering subsystem.
- 28: The last two digits ’28’ form the specific fault index.
Based on this breakdown, it’s clear that code P0128 refers to an engine coolant temperature issue where the temperature is below the thermostat regulating temperature.
Common DTC Codes and Their Meanings
Certain DTC codes occur more frequently than others. Below 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 valuable for identifying vehicle malfunctions, they don’t explain the underlying cause of the problem. A vehicle should undergo further diagnosis by a qualified mechanic to pinpoint and address the root cause.
Reading J1939 DTCs
A J1939 DTC comprises four fields that transmit information about the reported fault:
- Suspect Parameter Number (SPN): Represents the specific parameter with the fault. Each defined SPN can be used in a DTC.
- Failure Mode Identifier (FMI): Indicates the nature and type of error, such as value range violations (high or low), sensor shorts, incorrect update rate, or calibration errors.
- Occurrence Counter (OC): A counter that tracks the number of times the error condition occurs 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” chart. If CM has a value of “1”, a distinction must be made between three previously valid methods; this should be known by the system.
DTC Scanning Tools for Fleet Managers
Scanners that read DTCs, known as OBDII scanners, are widely available for purchase. However, for fleet managers overseeing multiple vehicles, manually checking each vehicle whenever the check engine light appears can be inefficient.
This underscores the importance of selecting a fleet management solution, like Samsara, that incorporates maintenance tools to streamline DTC identification. Samsara vehicle gateways connect directly to the vehicle’s OBDII port and transmit relevant data to the cloud, including DTCs. This means that if a DTC occurs, it automatically appears on your Samsara dashboard, or you can opt to receive notifications via alerts.
In situations where several fleet vehicles experience DTC issues, a telematics solution with a DTC tool can assist you in quickly pinpointing the problem, enabling you to take prompt action and schedule necessary maintenance.
Effortlessly Understand DTCs with Telematics
Seeing a check engine light illuminate on your dashboard without understanding the reason can be stressful. This is why adopting a telematics solution like Samsara, which aids in comprehending Diagnostic Trouble Codes (DTCs), is essential. By deciphering these codes, you can accurately identify your vehicle’s problem and take appropriate measures.
Samsara’s vehicle gateway seamlessly connects to a vehicle’s OBDII or J1939 port, enabling the transmission of DTC-related information directly to the Samsara dashboard. Furthermore, alerts can be activated in the event of a DTC occurrence using the “Vehicle Fault” alert feature. You can also leverage Samsara’s DTC filtering functionality to receive alerts for the DTCs most critical to your fleet.
To discover more about how Samsara can assist you in reading and addressing DTCs, sign up for a free trial or explore Samsara’s fleet maintenance solutions for further information.
