What 393.83B means in plain language
FMCSR 393.83B governs where your truck's exhaust pipe can discharge. Specifically, your exhaust system cannot be positioned so that it discharges below the fuel tank or fuel filler pipe opening.
Why does this matter? Fuel tank and filler pipes are critical safety components. If hot exhaust gases are directed below the fuel tank or near the filler pipe, you create a fire hazard. Fuel vapors near hot exhaust can ignite, putting your cargo, your truck, and your life at risk. This regulation exists to keep that risk contained by keeping exhaust well away from fuel system entry points.
The violation is straightforward: either your exhaust outlet is positioned too low relative to the fuel tank, or it's aimed in a direction that brings it into proximity with the filler pipe. Both scenarios violate the rule.
What our enforcement data actually shows
Across 13 million inspections in our database, 393.83B is cited infrequently—only 114 times all-time, ranking it #1380 out of 3,036 FMCSR codes by volume. That said, when it does get cited, the enforcement pattern is notable.
Our inspection records show a 20.2% out-of-service rate for 393.83B—meaning roughly 1 in 5 trucks cited for this violation were ordered off the road immediately. This is lower than the all-FMCSR average OOS rate of 31.4%, suggesting inspectors view this violation as less severe than average, though still serious enough to sideline vehicles in about one-fifth of cases.
In the last 12 months, we recorded 69 citations. Over the most recent 90 days, that number dropped to 20, indicating this violation remains relatively uncommon in current enforcement patterns.
Who gets cited most
Texas dominates the enforcement landscape for 393.83B. Across the last 180 days, our data shows 20 citations issued in Texas alone—far exceeding any other state. Of those 20, 7 resulted in out-of-service orders, yielding a 35.0% OOS rate in Texas. This is notably higher than the national 20.2% rate for this code, suggesting Texas inspectors take exhaust positioning seriously.
California and Indiana follow distantly, each with just 1–2 citations in the last 180 days, and neither resulting in out-of-service orders.
By carrier, our data shows fleets such as Jose Luis Delgado Castillo (USDOT 1064757) with 2 citations and Bundy Hauling (USDOT 2871389) with 2 citations. These remain isolated cases; no single carrier shows a pattern of repeat violations for this code.
How severe is this compared to similar codes
In the vehicle maintenance category, 393.83B sits well below the citation volume of related codes. For example, 393.9(a)—inoperable required lamps—has logged 660,737 citations with a 15.4% OOS rate. 396.3(a)(1), covering general inspection and repair maintenance, shows 236,919 citations with a 45.3% OOS rate.
Compare that to 393.78 (windshield condition defects), which has 157,894 citations and only a 0.3% OOS rate. Exhaust discharge positioning sits between these extremes: rare (114 all-time citations) but with an OOS rate (20.2%) that suggests inspectors treat confirmed violations as material safety defects.
How to avoid it
Preventing a 393.83B citation requires awareness of your truck's exhaust routing during pre-trip inspection and maintenance:
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Walk the truck and visually trace the exhaust path. Before every trip, look underneath your vehicle. Follow the exhaust pipe from the engine back to the tailpipe outlet. Ensure the outlet is clearly below and away from the fuel tank and filler neck. Any rust, corrosion, or misalignment that moves the exhaust outlet upward or toward the fuel system is a red flag.
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Check for corrosion and structural damage. Our inspection records show that 393.45B2UV (brake tubing/hoses inadequate) and 396.5B (fuel system leak) frequently appear alongside 393.83B citations. This pattern suggests that aging or damaged fuel and exhaust systems often occur together. During your pre-trip, examine the fuel tank mounting and brackets, and ensure the exhaust pipe isn't bent, rusted through, or sagging into proximity with fuel components.
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Be especially vigilant if your truck is older. The top vehicle makes cited for 393.83B include Freightliner, Kenworth, and Mack—vehicles that, while well-maintained by many operators, may experience exhaust or fuel system corrosion and sagging if not carefully inspected. Older trucks require more frequent attention to these systems.
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Have exhaust work done by qualified technicians. If your exhaust has been repaired, modified, or re-welded, ensure the work preserves the correct discharge position. Improper repair or aftermarket parts can inadvertently redirect the exhaust outlet toward the fuel system.
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Report sagging or repositioned exhaust immediately. If you notice during a trip that your exhaust has shifted or is hanging lower than usual, do not continue driving. Pull into a safe location and contact your fleet maintenance team. Continuing operation with a misdirected exhaust near your fuel tank risks fire and puts you out of service at the next inspection.
The data shows this violation is uncommon, but when it occurs, it merits immediate correction. A quick pre-trip walk-around that includes tracing your exhaust system is your best defense.