Seeing the ‘check engine light’ illuminate on your dashboard without understanding why can be unsettling. That’s why it’s crucial to have resources that help you decipher Diagnostic Trouble Codes (DTCs). Understanding these codes allows you to pinpoint vehicle issues and take appropriate action.
What is a DTC Code?
DTC stands for Diagnostic Trouble Code. It’s a fault code used to identify malfunctions in vehicles 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 diagnostic port.
A basic understanding of DTCs can be invaluable for both drivers and fleet managers. Continue reading to learn more about DTCs, how they function, and how to interpret them effectively.
The Origin of DTCs
DTCs are generated by a vehicle’s onboard diagnostics (OBD) system whenever a fault is detected. The OBD system diagnoses the fault and signals the DTC through visible warnings, such as activating the check engine light. It also enables external devices, like OBD scanners, to interact with the vehicle’s onboard computer system.
Over the years, various versions of OBD interfaces were in use, categorized as OBDI. These interfaces differed significantly between manufacturers. Today, two primary standards are used for reading 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) developed a standardized list of DTCs. This list includes fault codes that are common across all vehicle manufacturers.
Although OBDII is widely adopted, manufacturer-specific DTCs still exist. Manufacturers can create proprietary DTCs to supplement the universal code list when necessary for their vehicles. However, these manufacturer-specific codes can often be challenging to interpret without specialized mechanic knowledge.
OBDII vs. J1939 Compatibility: Which Does Your Vehicle Use?
OBDII Compatible:
- Cars and light trucks manufactured and sold in North America post-January 1, 1996, are mandated to be equipped with OBD-II.
J1939 Compatible:
- Most vehicles or equipment powered by diesel engines are J1939 compatible.
- It is the industry standard for the majority of heavy-duty trucks and buses.
If you are unsure whether your vehicle is OBDII or J1939 compliant, consult your vehicle’s manual, conduct an online search, or check with your dealership to confirm compatibility.
Decoding an OBD2 DTC: A Step-by-Step Guide
OBD2 DTCs are composed of five characters. Each character in the DTC provides distinct information about the vehicle’s problem.
The first character is always a letter, indicating the system affected:
- P (Powertrain): Relates to the engine, transmission, fuel system, and associated accessories.
- C (Chassis): Pertains to mechanical systems generally located outside the passenger compartment, such as steering, suspension, and braking.
- B (Body): Refers to parts mainly 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, specifying whether the code is standardized:
- 0: Denotes a generic, standardized SAE (Society of Automotive Engineers) code. All vehicles adhering to the OBDII standard use generic codes.
- 1: Indicates a manufacturer-specific code. These codes are unique to a specific vehicle brand or model and are generally less common.
- 2 or 3: Less frequent, and their meanings depend on the preceding letter. Mostly, 2 or 3 signify a manufacturer-specific code, with few exceptions.
The third character is also a digit, ranging from 1 to 8, pinpointing the malfunctioning subsystem:
- 1: Refers to the fuel or air metering system.
- 2: Relates to the fuel or air metering injection system.
- 3: Pertains to the ignition system.
- 4: Concerns the emission control system.
- 5: Relates to 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, read together as a two-digit number from 0 to 99, form the specific fault index. These digits precisely identify the exact vehicle problem.
If you are uncertain about your code’s meaning, consult your vehicle manufacturer or your vehicle manual for detailed explanations.
How to Interpret an OBD2 DTC
Each DTC corresponds to a particular vehicle malfunction, such as an engine misfire in cylinder one. Here’s how a common DTC, P0128, would be interpreted:
- P: Indicates the problem is within the Powertrain system.
- 0: Signifies that the code is not manufacturer-specific. It is standardized and interpretable according to SAE definitions.
- 1: Reveals the issue is specifically within the fuel and air metering subsystem.
- 28: The last two digits form the specific fault index.
Based on this breakdown, the code P0128 clearly points to an issue with the engine coolant temperature being below the thermostat regulating temperature.
Alt text: OBD2 P0128 code interpretation diagram showing each digit’s meaning: P – Powertrain, 0 – Generic, 1 – Fuel/Air Metering, 28 – Specific Fault Index. Text explains P0128 refers to engine coolant temperature below thermostat regulating temperature.
Common OBD2 DTCs to Be Aware Of
Certain DTCs occur more frequently than others. Here’s a list of common DTCs you might encounter and their corresponding malfunctions:
- P0442: Small leak detected in the 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 important to remember that while DTCs are helpful in identifying a vehicle malfunction, they do not explain the underlying cause of the problem. A vehicle should be further diagnosed by a qualified mechanic to pinpoint and address the root cause.
Understanding J1939 DTCs
A J1939 DTC comprises four fields that convey information about the reported fault. These include:
- Suspect Parameter Number (SPN): Represents the specific parameter that is malfunctioning. Each defined SPN can be used in a DTC.
- Failure Mode Identifier (FMI): Describes 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 an error condition occurs for each SPN and stores this count 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 a standard DTC structure. A CM value of “1” indicates distinctions between three previously valid methods, which should be known to the system.
DTC Scanning Tools for Fleet Management
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 comes on can be inefficient.
This is where selecting a fleet management solution like Samsara, which incorporates maintenance tools that streamline DTC identification, becomes essential. Samsara vehicle gateways connect directly to the vehicle’s OBDII port and transmit vehicle-related 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.
Alt text: Samsara dashboard interface displaying vehicle diagnostics information including DTC codes, vehicle gateway device plugged into OBDII port shown.
In cases where several vehicles in your fleet experience DTC issues, a telematics solution with a DTC tool can quickly help you identify the problem, allowing you to take prompt action to schedule maintenance. Using a DTC tool integrated into a fleet management system saves time and resources.
Simplify DTC Interpretation with Telematics
Seeing a check engine light without knowing the cause can be stressful. A telematics solution like Samsara helps simplify understanding Diagnostic Trouble Codes (DTCs), enabling you to identify vehicle issues and take appropriate actions efficiently.
Samsara’s vehicle gateway plugs directly into a vehicle’s OBDII or J1939 port, transmitting DTC-related information directly to the Samsara dashboard. Furthermore, alerts can be set up for DTC occurrences 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 discover more about how Samsara can assist you in reading and addressing DTCs, sign up for a free trial or explore more about Samsara’s fleet maintenance solutions.
