Seeing the ‘check engine light’ illuminate on your dashboard without knowing why can be stressful. That’s why having a solution that helps you understand OBD2 fault codes, also known as Diagnostic Trouble Codes (DTCs), is crucial. With this knowledge, you can identify your vehicle’s problem and take appropriate action.
What is an OBD2 Fault Code?
An OBD2 fault code, or DTC, is a diagnostic trouble code used to pinpoint malfunctions in a vehicle. While the malfunction indicator lamp (MIL), or check engine light, simply alerts drivers to a problem, a DTC specifies what the issue is and where it’s located. DTCs, also referred to as vehicle engine fault codes, can be accessed using a scanner that connects directly to a vehicle’s diagnostic port.
A basic understanding of OBD2 fault codes can be invaluable for drivers and fleet managers alike. Read on to learn more about DTCs, how they function, and how to interpret them.
The Origin of OBD2 Fault Codes
OBD2 fault codes are generated by a vehicle’s onboard diagnostic system (OBD) whenever a malfunction is detected. The OBD system diagnoses the fault and signals the DTC through visible warnings, such as activating the check engine light. It also allows external devices, like an OBD2 scanner, to interface with the vehicle’s onboard computer system.
Over the years, various versions of OBD interfaces were used, categorized as OBDI. These varied significantly between manufacturers. Today, two primary standards are used to read DTCs:
- OBD2: Best suited for light-duty and medium-duty vehicles.
- J1939: Ideal for heavy-duty vehicles and equipment.
With the implementation of OBDII, the Society of Automotive Engineers (SAE) developed a standardized list of DTCs containing fault codes common across all manufacturers.
While OBDII is widely adopted, manufacturer-specific DTCs still exist. Manufacturers can create proprietary DTCs to supplement the universal code list if needed, but these are often challenging to interpret without expert knowledge.
OBD2 vs. J1939: Compatibility
OBD2 Compatibility:
- Cars and light trucks manufactured and sold in North America after January 1, 1996, are mandated to be equipped with OBD-II.
J1939 Compatibility:
- Most vehicles or equipment powered by diesel engines are J1939 compatible.
- It’s the industry standard for most heavy-duty trucks and buses.
If you’re uncertain whether your vehicle is OBD2 or J1939 compliant, consult your vehicle’s manual, search online, or check with your dealership for confirmation.
Decoding an OBD2 Fault Code
OBD2 fault codes consist of five characters, each providing distinct information about the vehicle’s issue.
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): Pertains to mechanical systems outside the passenger compartment, like steering, suspension, and braking.
- B (Body): Refers to parts primarily within the passenger compartment area.
- U (Network): Indicates issues with the vehicle’s onboard computers and related systems.
The second character is a digit, typically 0 or 1, revealing if the code is standardized:
- 0: Signifies a generic, standardized SAE (Society of Automotive Engineers) code. All OBDII-compliant vehicles 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. Mostly, 2 or 3 indicate a manufacturer-specific code, with few exceptions.
The third character is also a digit, ranging from 1 to 8, pinpointing the specific subsystem at fault:
- 1: Refers to the fuel or air metering system.
- 2: Relates to the fuel or air metering injection system.
- 3: Points to the ignition system.
- 4: Indicates the emissions system.
- 5: Concerns vehicle speed controls and the idle control system.
- 6: Refers to the computer output circuit.
- 7 and 8: Indicate transmission-related issues.
The fourth and fifth characters are read together as a two-digit number from 0 to 99, known as the specific fault index. These digits identify the precise nature of the vehicle problem.
If you’re unsure about your code’s meaning, consult your vehicle manufacturer or your manual for detailed explanations.
Image: OBD2 connector port in a vehicle, used to connect diagnostic scanners for reading fault codes.
Interpreting OBD2 DTCs: An Example
Each OBD2 fault code corresponds to a specific vehicle malfunction, such as an engine misfire in cylinder one. Let’s break down a common DTC example: P0128.
- P: Indicates the problem is within the powertrain system.
- 0: Shows this is a generic, non-manufacturer-specific code, standardized by SAE.
- 1: Reveals the issue is specifically within the fuel and air metering subsystem.
- 28: The specific fault index.
Therefore, the code P0128 translates to a problem with the engine coolant temperature being below the thermostat regulating temperature.
Common OBD2 Fault Codes
Some OBD2 fault codes are more frequently encountered than others. Here’s a list of common DTCs and their corresponding malfunctions:
- P0442: Small Evaporative Emission System Leak Detected.
- P0606: PCM (Powertrain Control Module) Malfunction. Also known as ECM (Engine Control Module) malfunction.
- P0101: Mass Air Flow (MAF) Sensor 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 OBD2 fault codes help pinpoint malfunctions, they don’t explain the root cause. Further diagnosis by a qualified mechanic is needed to identify and address the underlying issue.
Image: Dashboard view showing the illuminated check engine light, signaling a potential issue detected by the OBD system.
Understanding J1939 DTCs
A J1939 DTC contains four fields that transmit information about the reported fault:
- Suspect Parameter Number (SPN): Represents the specific parameter with an error. 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 frequency of the error condition for each SPN and stores it even after the error is no longer active.
- SPN Conversion Method (CM): Defines byte alignment within the DTC. A value of “0” represents the method shown in standard documentation. A value of “1” indicates distinctions between previously valid methods, which should be known to the system.
DTC Scanning Tools for Fleet Management
Scanners that read OBD2 fault codes, known as OBDII scanners, are widely available for purchase. However, manually checking each vehicle every time the check engine light comes on can be inefficient for fleet managers responsible for multiple vehicles.
This is where a fleet management solution like Samsara, incorporating maintenance tools, becomes invaluable. Samsara vehicle gateways plug directly into the vehicle’s OBDII port and transmit OBD2 data, including DTCs, to the cloud. This means DTCs automatically appear on your Samsara dashboard, or you can opt to receive notifications via alerts.
When multiple vehicles in your fleet experience DTC issues, a telematics solution with a DTC tool helps quickly identify problems, enabling you to schedule necessary maintenance promptly.
Image: A mechanic using an OBD2 scanner connected to a car’s OBD2 port to diagnose and read diagnostic trouble codes.
Simplify DTC Management with Telematics
Seeing a check engine light without understanding the cause can be concerning. A telematics solution like Samsara helps decipher OBD2 fault codes, enabling you to identify vehicle problems and take appropriate action.
Samsara’s vehicle gateway connects directly to a vehicle’s OBDII or J1939 port, transmitting DTC information to the Samsara dashboard. Vehicle fault alerts can be activated to notify you of DTC occurrences. Samsara’s DTC filtering feature allows you to prioritize alerts for the most critical DTCs for your fleet.
To learn more about how Samsara can assist with reading and managing OBD2 fault codes, sign up for a free trial or explore Samsara’s fleet maintenance solutions.
