TruckCodex

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 Operating a CMV while ill or fatigued Unsafe Driving 1,208,246
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 216,967
396.17(c) No proof of periodic inspection Vehicle Maintenance 198,331
392.2-SLLSR Operating a CMV while ill or fatigued Unsafe Driving 194,362
393.47E Slack adjuster defective Vehicle Maintenance 184,958
393.9 Inoperable Required Lamp Vehicle Maintenance 182,266
393.11 Lighting devices/reflectors Vehicle Maintenance 180,162
393.78 Windshield condition defective Vehicle Maintenance 158,863
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 158,425
393.47(e) Slack adjuster defective 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.

Related pages

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.

More to explore

OBD-II Codes
Light- and medium-duty diagnostic codes
J1939 Codes
Heavy-duty SPN/FMI codes
VIN Lookup
Decode any 17-character VIN
Violation Codes
FMCSR roadside citation codes
NHTSA Recalls
NHTSA active recall campaigns

Data sources & freshness

TruckCodex aggregates official public-sector datasets. See the Source registry for dataset-level coverage and the Freshness log for last-import timestamps.

FMCSA

Census, SAFER, SMS, Licensing & Insurance (L&I), roadside inspections, crashes, and authority history.

Refreshed daily.
NHTSA

Vehicle recall campaigns, defect investigations, and consumer safety complaints (SCRS).

Refreshed daily.
EIA

Retail diesel and gasoline price history and state fuel-tax tables.

Refreshed weekly.
Transport Canada

Cross-border carrier registry and Canadian recall campaigns where applicable.

Refreshed weekly.

TruckCodex is an independent aggregator; it is not affiliated with FMCSA, NHTSA, EIA, or Transport Canada. Always verify compliance-critical information directly with the originating agency.