What 393.83(e) means in plain language
This regulation is straightforward: on a truck or truck-tractor, the exhaust system must discharge its gases at the rear of the cab — not somewhere in the middle of the vehicle, not beneath the cab doors, and not anywhere that routes fumes toward the driver or occupants.
The intent is to keep exhaust away from the people inside the cab and away from areas where cargo could be damaged or personnel could be exposed. If your exhaust pipe is broken, rerouted, or has a disconnected section that vents somewhere other than the proper rear-of-cab exit point, you are out of compliance — regardless of whether the engine is running clean.
This is a vehicle maintenance issue, not an emissions standard. Inspectors are looking at where your exhaust exits the system, not what comes out of it.
What our enforcement data actually shows
Across our 13 million+ inspection records, 393.83(e) has generated 3,923 all-time citations, placing it at #392 out of 3,036 FMCSR codes by citation volume. That puts it in the upper half of all tracked codes — not a fringe violation, but also not among the most common maintenance citations officers write.
The out-of-service picture is important context for anyone who just got cited. Of those 3,923 citations, only 54 resulted in an OOS order — that is a 1.4% OOS rate. Compare that to the all-FMCSR average OOS rate of 31.4% across all codes in our database, and you can see that 393.83(e) almost never takes a truck off the road on its own. The regulation is not OOS-eligible under standard FMCSA criteria, which explains why 3,869 of those citations — the overwhelming majority — resulted in a citation only, with the driver continuing on their way.
That said, the recent trend is notable: our inspection records show zero citations in the last 90 days and zero in the last 12 months. This code appears to have faded significantly from active enforcement in the current period, though the all-time volume confirms it was a real and regularly-cited issue in prior years.
Who gets cited most
The top-carrier data in our database tells a clear geographic story. Every carrier in the top 10 by citation count is a Mexican-domiciled motor carrier, which reflects where this violation has historically been concentrated — at and near the southern border, where cross-border commercial traffic is heaviest and where older equipment configurations are more common.
Our data shows fleets such as OPERADORA DE TRANSPORTE INTERNACIONAL SA DE CV (USDOT 683428) with 40 citations and AUTOTRANSPORTES ROMEDU SA DE CV (USDOT 1148259) with 34 citations leading all carriers in all-time 393.83(e) volume. This pattern does not imply negligence on the part of any carrier — it reflects enforcement concentration in specific corridors and equipment types.
On the vehicle side, our inspection records show Freightliner units accounting for 589 citations, Kenworth for 367, and International for 319. These are the dominant platforms in long-haul trucking, so their presence at the top of this list is partly a function of how many of them are on the road. Still, if you operate any of these makes — particularly older model years where exhaust routing hardware may have seen more wear — this violation is worth watching on your pre-trip.
How severe is this compared to similar codes
To understand where 393.83(e) sits in the broader Vehicle Maintenance category, compare it to a few peer codes from the same regulatory family.
393.9(a) — Inoperable required lamps has 660,737 all-time citations in our database — more than 168 times the volume of 393.83(e) — with a 15.4% OOS rate. Lamp violations are among the most frequently cited maintenance issues officers encounter, and their OOS rate is far higher than the 1.4% seen for 393.83(e).
393.11 — Lighting devices/reflectors shows 179,734 citations and a 1.8% OOS rate. This code is much closer to 393.83(e) in OOS risk profile — both sit well below the 31.4% all-FMCSR average — but the lighting violation is cited roughly 46 times more often.
393.78 — Windshield condition defective carries 157,894 citations and a 0.3% OOS rate, making it one of the lowest-OOS-risk codes in the category. Even compared to that benchmark, the 1.4% rate for 393.83(e) is only modestly higher.
The takeaway: 393.83(e) is a lower-severity citation in terms of OOS risk, but it does carry CSA points and it does appear on your inspection report — which matters for your carrier's Safety Measurement System (SMS) profile over time.
How to avoid it
Because this violation is about physical exhaust routing, it is almost always detectable during a thorough pre-trip if you know what to look for. The top vehicle makes in our data — Freightliner, Kenworth, and International — all use stack or side-exit exhaust configurations that can develop cracks, loose clamps, or broken hangers over time. Here is what to do before every trip:
- Walk the exhaust path end to end. Start at the exhaust manifold and trace the system to the point where it exits. Any section that is venting outside the rear-of-cab exit point is a potential 393.83(e) violation. Look for soot streaks on frame rails, cab panels, or the side of the vehicle as a fast indicator of an unintended discharge point.
- Check flex sections and clamps. Flexible couplings between rigid exhaust sections are common failure points. A cracked flex section can redirect exhaust gases sideways or downward before they reach the exit point. Squeeze or flex these sections by hand if safe to do so — any audible exhaust escape is a red flag.
- Inspect exhaust hangers and brackets. A dropped or sagging exhaust pipe may no longer align with its designed exit point. Look underneath the cab for any pipe that appears lower than it should be or is no longer secured to its mounting bracket.
- Look for heat damage or discoloration on cab panels. If exhaust is venting near the cab body rather than at the rear, you will often see paint discoloration, bubbling, or carbon buildup near the problem area long before an inspector notices it at the roadside.
- On Kenworth and Peterbilt units with vertical stacks, confirm the stack is fully secured and the rain cap is functional. A cracked stack base can allow discharge at the wrong point on the cab structure.
This is a fixable violation with inexpensive parts and shop time. Catching it during your pre-trip costs nothing. Catching it at a weigh station costs you a citation and a line on your inspection report.