What 393.91 means in plain language
FMCSR 393.91 addresses a specific vehicle maintenance requirement that inspectors enforce during roadside checks. The regulation requires commercial vehicles to meet particular standards that fall under the broader vehicle maintenance category. When an inspector cites you for 393.91, they are documenting that your vehicle did not meet the condition required by this code at the time of inspection.
This is a vehicle maintenance violation, not a documentation or operational issue. It means something on your truck itself—a component, system, or condition—was not in the state the regulation requires. The violation is noted in your inspection record, and depending on the severity and the inspector's judgment, you may or may not be placed out of service on the spot.
What our enforcement data actually shows
Across our database of 13 million+ roadside inspection records, 393.91 has generated 128 all-time citations. In the last 12 months and last 90 days, we have recorded zero citations for this code. This makes 393.91 one of the rarest citations in the FMCSR enforcement universe—it ranks #1356 out of 3,036 FMCSR codes by citation volume.
Of the 128 all-time citations, 68 resulted in out-of-service (OOS) orders, and 60 did not. This produces an OOS rate of 53.1% for 393.91. That is significantly higher than the all-FMCSR average OOS rate of 31.4%, meaning inspectors are more than 1.7 times as likely to remove a vehicle from service when they cite this code compared to the typical FMCSR violation. This elevated OOS rate reflects the seriousness with which enforcement is applied when 393.91 is cited.
Who gets cited most
Our inspection records show that citations for 393.91 are concentrated among specific fleets. American Transportation Systems (USDOT 1151228) and Kings Canyon Unified School District (USDOT 2607803) each have 4 citations for this code in our all-time database. Lutheran Secondary School of Clark County, Shoreline Unified School District, Windstar Lines Inc, and Coach Quarters Entertainment Transportation LLC each account for 3 citations.
The presence of school districts and charter bus operators among the top cited carriers reflects the vehicle types most commonly involved: motorcoaches and passenger transport vehicles. When you look at the top vehicle makes cited for 393.91, THMS leads with 13 citations, followed by BLUB with 7, and FORD with 5. This pattern—THMS, BLUB, and FORD—indicates that inspectors are most often documenting this violation on specific vehicle platforms, likely those used in passenger or specialized transport operations.
How severe is this compared to similar codes
To understand where 393.91 sits in the enforcement landscape, we can compare it to other vehicle maintenance codes. Code 393.9(a), Inoperable required lamps, has generated 660,737 citations with a 15.4% OOS rate—vastly more frequent but much less likely to result in removal from service. Code 396.3(a)(1), Inspection/repair/maintenance general, has 236,919 citations with a 45.3% OOS rate, which is closer to 393.91's severity but represents far heavier enforcement volume.
Most relevant for understanding the enforcement environment is code 393.9, Inoperable Required Lamp, which has 180,097 citations and a 6.9% OOS rate. Despite being in the same lighting/maintenance domain and much more commonly cited, 393.9 results in OOS placement less than one-seventh as often as 393.91. This tells us that when inspectors cite 393.91 specifically, they view the condition as more serious than even the typical lamp violation.
How to avoid it
Because 393.91 citations are extremely rare in our dataset (zero in the last year), standard pre-trip inspection discipline is your strongest defense. Here are concrete steps:
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Perform a full pre-trip inspection before every shift. Walk around your vehicle, check all exterior components, lights, fasteners, and visible systems. Document what you check and note any defects immediately.
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Pay special attention to your vehicle make and model. Our data shows THMS, BLUB, and FORD vehicles account for the highest citation counts for this code. If you operate one of these platforms, research known inspection points for your specific year and model, and add them to your checklist.
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If you operate a motorcoach or passenger vehicle, treat maintenance as critical. The top cited carriers in our data are passenger transport operators. Passenger vehicle regulations are enforced more strictly, and inspectors are likely to be more thorough. Do not defer any maintenance issues.
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Address maintenance defects the same day they appear. Do not drive with a known defect hoping to fix it later. The 53.1% OOS rate for this code means inspectors will remove you from service, which stops your revenue immediately and can trigger additional penalties.
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Keep maintenance records and receipts organized. When a defect is found and repaired, document the repair with date, mileage, and parts. This creates a defensible record if there is any dispute about when a condition developed.
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Schedule preventive maintenance on a strict calendar. Do not run your vehicle to failure. Regular servicing at manufacturer-recommended intervals prevents the roadside surprises that lead to citations.