What 393.83D means in plain language
Code 393.83D applies to buses powered by diesel, natural gas, or other fuels besides gasoline. The regulation requires that the exhaust system on these vehicles be properly maintained and functional. This includes the muffler, exhaust pipes, and any emission control components that route and treat engine exhaust gases before they exit the vehicle.
When an inspector cites you for 393.83D, they've found that your bus's exhaust system does not meet Federal Motor Carrier Safety Regulation standards. This could mean visible damage to the exhaust components, leaks in the system, missing or improperly secured parts, or evidence that exhaust is escaping where it shouldn't. The violation focuses specifically on the condition and integrity of these components, not on the chemical composition of the emissions themselves.
What our enforcement data actually shows
Across our 13 million inspection records, 393.83D is a relatively uncommon citation—only 43 all-time citations place it at rank #1675 of 3,036 FMCSR codes by enforcement volume. However, what makes this code notable is its severity: the out-of-service rate is 51.2%, meaning that just over half of all 393.83D citations result in the vehicle being immediately removed from service. This rate is significantly higher than the all-FMCSR average OOS rate of 31.4%, indicating that when inspectors cite this code, the exhaust defect typically prevents the bus from operating legally until repairs are made.
In the last 12 months, we recorded 25 citations for 393.83D. Over the last 90 days, that number dropped to 8 citations. The monthly trend shows a spike in March 2026 with 7 citations (4 resulting in OOS), suggesting this is not a uniformly distributed violation across the year.
Who gets cited most
Our inspection records show that 393.83D citations are highly concentrated in two states over the last 180 days. Texas leads with 11 citations, though the out-of-service rate there is 36.4%—lower than the national OOS rate for this code. Illinois recorded only 2 citations, but both resulted in vehicles placed out of service, representing a 100.0% OOS rate. This variation suggests that Texas inspectors and operators may encounter exhaust issues more frequently, or that some Texas citations involve less severe defects.
Among carriers in our database, Tornado Bus Company (USDOT 565859) appears most frequently, with 11 all-time citations for 393.83D. Several other operators—Transportes Regiomontanos Inc, All Valley Charters LLC, Turistica Merlo Inc, Juan Ramon Morales Mendoza, Jose Luis Gutierrez, and Flo Charter LLC—each received 2 citations. Our data reflects these citation patterns; no assertion about cause or negligence should be inferred.
How severe is this compared to similar codes
393.83D sits within the vehicle maintenance category alongside other codes, but its enforcement volume and severity differ markedly. For perspective, 393.9(a) (Inoperable required lamps) has generated 660,737 citations with a 15.4% OOS rate—far more common but much less likely to result in immediate vehicle removal. Code 396.3(a)(1) (Inspection/repair/maintenance general) shows 236,919 citations with a 45.3% OOS rate, placing exhaust defects in a middle tier for both frequency and severity. The windshield code 393.78 has 157,894 citations but only a 0.3% OOS rate, showing that exhaust issues are treated with greater urgency by enforcement.
How to avoid it
Exhaust system failure often accompanies other maintenance breakdowns. In the last 90 days, our data shows that fuel system leaks (code 396.5B) co-occurred with 393.83D in 3 shared inspections, and brake-related codes appeared in 1 each. This pattern suggests that carriers with systemic maintenance gaps may face multiple citations at once.
Before every shift, perform these driver-level checks:
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Walk around the rear of the bus and inspect visible exhaust components. Look for rust, holes, cracks, or hanging sections of pipe. Feel for loose clamps or hangers. If any part is visibly damaged or rattling, report it immediately; do not operate the vehicle.
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Listen for unusual exhaust noise during startup and idle. A sudden loud or rattling sound from the rear often signals internal or loose components. Stop and report it rather than risk an OOS citation downroad.
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Check that the muffler and tail pipe are fully secured and do not touch the frame or fuel tank. Chafing or contact can lead to cracks and leaks.
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On vehicles equipped with diesel particulate filters or other emissions gear, ensure no warning lights are illuminated on the dashboard. These systems are integral to the exhaust definition under 393.83D.
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Report any visible soot, dripping liquid, or strong odor from the exhaust system immediately to your safety manager. These are early signs of system failure.
Fleet safety managers should ensure that exhaust system inspections are part of every DVIR (Driver Vehicle Inspection Report) cycle and that repair turnaround is fast. Our data shows that Texas operators encounter this violation more than others; if your fleet operates there, consider more frequent pre-trip emphasis on exhaust condition.