Diagnostic Trouble Codes (DTCs) are standardized alphanumeric or numeric codes used by vehicles to report issues. They are generated by onboard diagnostic systems like OBD-II (used in light- and medium-duty vehicles) and J1939 (used in heavy-duty commercial vehicles and equipment). DTCs play a critical role in modern vehicle diagnostics, enabling maintenance teams to identify issues efficiently and resolve them before they escalate.
What Are Fault Codes?
Fault codes are a vehicle’s internal language for signaling that something is wrong. These codes are stored in the vehicle’s diagnostic system and can indicate issues with the engine, transmission, emissions, brakes, sensors, and more.
In OBD-II, codes are alphanumeric (e.g., P0301).
In J1939, codes use numeric SPN (Suspect Parameter Number) and FMI (Failure Mode Identifier) formats (e.g., SPN 157 FMI 18).
Both systems trigger alerts (e.g., warning lights or fault messages) when a code is logged.
How DTC Codes Work?
When a problem is detected within the vehicle's systems, a DTC code is triggered and logged in the vehicle diagnostic system. This code provides a detailed description of the fault and often activates the Check Engine Light or other warning indicators to alert the driver.
For example:
The vehicle diagnostic system continuously monitors components and generates codes based on the type and severity of the issue.
Types of DTC Codes
OBD-II Codes
Generic Codes (P0, P2, etc.)
Apply across all manufacturers; cover systems like emissions and fuel injection.
Manufacturer-Specific Codes (P1, P3, etc.)
Proprietary to OEMs; provide deeper diagnostics beyond the standard set.
J1939 Codes
SPN (Suspect Parameter Number): Identifies the parameter or component where the issue occurred.
FMI (Failure Mode Identifier): Describes the type of fault (e.g., short circuit, data out of range).
Note
Example: SPN 651 FMI 5 = Turbocharger
Boost Pressure - Current Below Normal.
J1939 codes are used in
trucks, buses, trailers, off-highway equipment.
Structure of DTC Codes
OBD-II Format
Each DTC code consists of five characters:
- First Character: Indicates the vehicle system (e.g., P for Powertrain, C for Chassis).
- Second Character: Distinguishes between generic (0) and manufacturer-specific (1) codes.
- Remaining Characters: Specify the nature of the issue, such as the location and type of fault.
Note
For example, P0420 represents an emissions system issue,
specifically a catalyst system efficiency fault.
J1939 Format
SPN (Suspect Parameter Number): This tells you which part or system is having an issue — like the accelerator pedal, engine temperature, or brake system.
FMI (Failure Mode Identifier): This describes what kind of problem happened — for example, a voltage that’s too high, a signal that’s missing, or a part that isn’t responding.
Occurrence Count (OC): This shows how many times the issue has occurred.
Conversion Method (CM): A technical field that tells the system how to interpret the SPN — usually not something you need to worry about.
Note
For example, SPN 91 FMI 3 (SPN 91 = Accelerator Pedal Position
FMI 3 = Voltage above normal)