Diagnostic Trouble Code Reference

Look up OBD-II, J1939, and FMCSR diagnostic trouble codes used in vehicle inspections, emissions testing, and fleet maintenance.

Diagnostic trouble codes (DTCs) are the standardized fault numbers a vehicle's electronic control modules store when a sensor or system parameter falls out of expected range. This reference covers the three code families used in U.S. trucking: OBD-II for light- and medium-duty vehicles (P, B, C, U codes under SAE J2012), J1939 SPN/FMI for heavy-duty Class 7 and 8 commercial trucks, and the FMCSR violation codes roadside inspectors cite under 49 CFR. Use the family cards below to drill into any specific code, or scan the most-cited table to see which violations actually show up in FMCSA roadside inspections.

OBD-II Diagnostic Codes

224 on file

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 on file

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

Violation codes most frequently cited across FMCSA roadside inspections.

12 codes

Most cited violation codes across FMCSA inspections
Code	Description	Category	Times Cited
392.2	Violation of Local Laws - Explain:	Unsafe Driving	1,208,369
393.9(a)	Inoperable required lamps	Vehicle Maintenance	660,737
396.17C-PI	Operating a CMV without documentation of a periodic inspection.	Vehicle Maintenance	260,410
396.3(a)(1)	Inspection, repair and maintenance of parts and accessories	Vehicle Maintenance	236,919
392.2-SLLSR	State/Local Laws - State registration/tag violation.	Unsafe Driving	235,514
393.47E	Brake Out of Adjustment - Roto, Clamp (Short & Long), DD-3, or Bolt	Vehicle Maintenance	215,385
396.17(c)	Operating a CMV without proof of a periodic inspection	Vehicle Maintenance	198,331
393.9	Inoperable Required Lamp	Vehicle Maintenance	195,132
393.75A3-TAOL	Tires - All others, leaking or inflation less than 50% of the maximum inflation pressure.	Vehicle Maintenance	189,594
393.11	No or defective lighting devices or reflective material as required	Vehicle Maintenance	183,085
393.78	Wipers - Inoperative / missing / damaged wipers.	Vehicle Maintenance	165,312
393.47(e)	Brake Out of Adjustment - Roto, Clamp (Short & Long), DD-3, or Bolt	Vehicle Maintenance	157,499

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.

In FMCSA roadside inspections, active fault codes related to brakes, emissions systems, or lighting can contribute to out-of-service orders. Inspectors may connect to a vehicle's OBD-II or J1939 diagnostic port during Level I and Level II inspections.

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.

Frequently asked questions

What is a diagnostic trouble code? ▾

A Diagnostic Trouble Code (DTC) is a standardized fault number stored by a vehicle's electronic control module when a sensor reading or system parameter falls outside its expected range. DTCs are read with a scan tool plugged into the diagnostic port and used by technicians to localize mechanical, electrical, and emissions faults.

What is the difference between OBD-II and J1939? ▾

OBD-II is the standard on light- and medium-duty vehicles (cars, pickups, vans) sold in the U.S. since 1996. J1939 is the standard on heavy-duty Class 7 and 8 commercial vehicles — long-haul trucks, transit buses, off-highway equipment. They use different connectors, different protocols, and different code formats. A heavy-duty diesel truck almost always speaks J1939 (SPN/FMI), not OBD-II.

Can a diagnostic code keep my truck out of service? ▾

Sometimes. During FMCSA Level I and II roadside inspections, an active DTC related to brakes, emissions aftertreatment (DPF, DEF, SCR), or required lighting can contribute to an out-of-service order under 49 CFR 393 or 396. Pending or historical codes by themselves usually do not. The inspector's discretion and the inspection level both matter.

Are these code descriptions the official SAE definitions? ▾

No. SAE J2012 (OBD-II) and J1939 (heavy-duty) are licensed standards — the canonical definitions are only available through SAE's Digital Annex or licensed scan-tool databases. The descriptions on TruckCodex are summarized in plain English for reference. For repair work, always cross-check with a licensed scan tool or the OEM service manual.

How often is the code reference updated? ▾

The standard SAE codes change rarely — usually only when SAE publishes a new revision of J2012 or J1939. Manufacturer-specific codes (P1xxx, P3xxx on OBD-II) get added as we encounter them. Inspection-citation counts on the hub refresh nightly with the FMCSA roadside-inspection feed.

Code System

OBD-II Diagnostic Codes

P / B / C / U code families.

Code System

J1939 SPN/FMI Codes

Heavy-duty diesel diagnostic codes.

Reference

FMCSA Violation Codes

Violation codes by BASIC category.

Database

NHTSA Safety Recalls

Active recall campaigns for commercial vehicles.