What 393.83(g) means in plain language
This regulation targets where your exhaust system releases its gases. Specifically, it prohibits the exhaust from exiting the system at any point that is forward of or directly beneath the cab where the driver sits — or beneath a sleeper berth if your truck has one.
The concern is straightforward: exhaust fumes routed in those directions can enter the cab or sleeper through gaps, vents, or airflow patterns, creating a carbon monoxide and air quality hazard for the occupants. The rule is essentially saying your exhaust needs to go away from the people in the truck, not toward or under them.
For most modern over-the-road tractors, the exhaust stack exits vertically behind or beside the cab, which keeps the truck well clear of this violation. The citations tend to appear when exhaust components have corroded, separated, been improperly repaired, or when a vehicle has been modified in a way that relocated the discharge point without accounting for this requirement.
What our enforcement data actually shows
Across our database of 13 million+ inspections, 393.83(g) has generated 22,015 all-time citations, ranking it #125 out of 3,036 FMCSR codes by citation volume. That puts it in the top 5% of all cited codes — a meaningful enforcement presence even if it doesn't dominate the inspection landscape.
The out-of-service picture is notably favorable for drivers. Of those 22,015 citations, 850 resulted in an out-of-service order and 21,165 did not, producing an OOS rate of 3.9%. To put that in context, the all-FMCSR average OOS rate across all codes is 31.4%. At 3.9%, this code runs more than 27 percentage points below that average — meaning that in the vast majority of cases, an inspector writes the citation and sends the driver on their way rather than parking the truck.
This code is not OOS-eligible under standard criteria, which aligns with what the raw numbers show. Being cited hurts your inspection record and your carrier's CSA scores, but it almost never stops your load from moving that day.
One detail worth paying attention to: our inspection records show zero citations in the last 12 months and zero in the last 90 days. The 22,015 citations in the database represent historical enforcement activity, and active citation volume has gone quiet in recent reporting periods. That said, the violation itself hasn't disappeared from the books — inspectors can and do cite it when they find a discharge point that isn't compliant.
Who gets cited most
The carrier distribution in our data tells a clear story about where this violation concentrates. The top carriers by all-time citation count are dominated by cross-border operators. Our data shows fleets such as RUBEN CARLOS TREVINO SANCHEZ (USDOT 1649689) with 115 citations, TRANSPORTE INTERNACIONAL LOPEZ OCHOA SA DE C V (USDOT 1041907) with 114 citations, and JESUS MA VALDEZ GARCIA (USDOT 2534784) with 106 citations leading the list. Several other Mexico-domiciled carriers also appear in the top ten, including TRANSPORTES REFRIGERADOS GC XPRESS SA DE CV with 95 citations and AUTOTRANSPORTES VARELA DAVILA SA DE CV with 71 citations.
This pattern suggests that border-crossing inspection points — where vehicles entering from Mexico receive Level I or focused inspections — are a significant source of 393.83(g) citations. Vehicles that may not have been inspected as frequently in their home country, or that carry older or modified exhaust configurations, are more likely to surface this defect at a U.S. port of entry or border-state inspection station.
On the vehicle side, our inspection records show Freightliner (FRHT) leading all makes with 2,468 citations, followed by Kenworth (KW) at 1,734 and Freightliner variants labeled FREIGHTLIN at 1,145. Peterbilt (PTRB) and International (INTL) also appear in the top cited makes. Given that Freightliner and Kenworth together dominate the North American fleet, their volume here is largely proportional to their market share — but it does mean drivers of those platforms should be especially deliberate about exhaust routing during pre-trip.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.83(g) is a relatively minor citation by any measure. Consider a few peers from our database:
393.9(a) — Inoperable required lamps has been cited 660,737 times — roughly 30 times the volume of 393.83(g) — and carries a 15.4% OOS rate. Lamp violations are both far more common and more likely to park your truck.
396.3(a)(1) — Inspection/repair/maintenance (general) has 236,919 citations and a 45.3% OOS rate. That code runs more than 41 percentage points higher OOS rate than 393.83(g), and inspectors use it broadly as a catch-all for maintenance failures.
393.78 — Windshield condition defective shows 157,894 citations at a 0.3% OOS rate, making it one of the few Vehicle Maintenance codes with a lower OOS rate than 393.83(g). It's cited far more often but almost never results in an out-of-service order.
In short, 393.83(g) sits in the middle ground: cited often enough to matter for CSA scoring, but with an OOS rate that places it well below the codes that actually shut down operations.
How to avoid it
Because exhaust problems are progressive — they typically worsen over time from vibration, heat cycling, and corrosion — your pre-trip inspection is the primary defense. Here is what to check:
- Walk the exhaust path from manifold to tip. On your pre-trip, trace where the exhaust pipe runs from the engine side toward the exit point. If any section has shifted, separated at a joint, or shows signs of a repair that redirected the pipe, flag it before departure.
- Check for rust, cracks, and loose clamps at each joint. Freightliner, Kenworth, and Peterbilt platforms — the most-cited makes in our data — use flex sections and clamp connections that are common failure points. A loose clamp can allow a pipe to drop or rotate, changing where the exhaust exits.
- Inspect beneath the cab and sleeper on every pre-trip. Get low enough to see whether any pipe is terminating below the cab floor or routed forward. This is the geometry the regulation targets, and it's not always visible from a standing position.
- After any exhaust repair, verify the exit point. If your truck has had a muffler, flex pipe, or stack replaced or repositioned, confirm the repair shop returned the discharge point to its original location — not forward and not directly below the cab or sleeper.
- Note cross-border inspection risk. Our data shows that carriers operating in international cross-border lanes generate a disproportionate share of these citations. If you're running loads that cross into U.S. ports of entry, treat the exhaust system as a high-priority check, not an afterthought.