Diagnostic Trouble Code Reference
Look up OBD-II, J1939, and FMCSR diagnostic trouble codes used in vehicle inspections, emissions testing, and fleet maintenance.
OBD-II Diagnostic Codes
On-Board Diagnostics II codes for light- and medium-duty vehicles. Covers powertrain (P), body (B), chassis (C), and network (U) fault categories standardized under SAE J2012.
Examples: P0300, P0420, P0171
J1939 SPN/FMI Codes
94 codes
SAE J1939 diagnostic codes for heavy-duty trucks, buses, and off-highway equipment. Uses SPN (Suspect Parameter Number) paired with FMI (Failure Mode Identifier) over CAN bus.
Examples: SPN 91 FMI 1, SPN 100 FMI 1, SPN 110 FMI 0
FMCSR Violation Codes
Federal Motor Carrier Safety Regulation codes cited during roadside inspections. Mapped to BASIC categories used in carrier safety scoring.
Examples: 393.47, 395.3, 396.3
Most Cited Codes in Inspections
| Code | Description | Category | Times Cited |
|---|---|---|---|
| 392.2 | Operating a CMV while ill or fatigued | Unsafe Driving | 1,208,164 |
| 393.9(a) | Inoperable required lamps | Vehicle Maintenance | 660,737 |
| 396.3(a)(1) | Inspection/repair/maintenance - general | Vehicle Maintenance | 236,919 |
| 396.17C-PI | No proof of periodic inspection | Vehicle Maintenance | 212,081 |
| 396.17(c) | No proof of periodic inspection | Vehicle Maintenance | 198,331 |
| 392.2-SLLSR | Operating a CMV while ill or fatigued | Unsafe Driving | 191,232 |
| 393.47E | Slack adjuster defective | Vehicle Maintenance | 180,363 |
| 393.9 | Inoperable Required Lamp | Vehicle Maintenance | 180,097 |
| 393.11 | Lighting devices/reflectors | Vehicle Maintenance | 179,734 |
| 393.78 | Windshield condition defective | Vehicle Maintenance | 157,894 |
| 393.47(e) | Slack adjuster defective | Vehicle Maintenance | 157,499 |
| 393.75A3-TAOL | Tires - All others leaking or inflation less than 50% of the maximum inflation pressure on tire not equipped with ATIS | Vehicle Maintenance | 155,660 |
About Diagnostic Trouble Codes
Diagnostic Trouble Codes (DTCs) are standardized fault codes stored by a vehicle's electronic control modules when a sensor reading or system parameter falls outside its expected range. Technicians read these codes using scan tools to diagnose mechanical, electrical, and emissions issues.
During FMCSA roadside inspections, inspectors may note active fault codes as part of a Level I or Level V inspection. Certain DTCs — especially those related to emissions systems — can result in out-of-service orders if the vehicle fails to meet standards.
TruckCodex maintains a reference database of OBD-II, J1939, and FMCSR codes with plain-language descriptions, common causes, and links to the relevant violation codes when applicable.