What 393.83(d) means in plain language
This citation applies to buses that run on fuels other than gasoline—typically diesel, natural gas, or propane. The regulation requires that the exhaust system on these vehicles meet specific standards for how exhaust is discharged from the engine.
In practical terms, this means your bus's exhaust pipe, muffler, and related components must be properly installed, securely fastened, and positioned so that exhaust gases exit safely without creating a hazard to other vehicles or road users. An inspector citing you for 393.83(d) found that something about your exhaust system—whether loose, damaged, leaking, or improperly routed—fell short of that standard.
This is a defect specific to non-gasoline buses. If you operate a gasoline-powered bus or a different vehicle type, you would be cited under a different code.
What our enforcement data actually shows
Across 13 million roadside inspections in our database, 393.83(d) has generated 355 all-time citations. Over the last 12 months and last 90 days, we have recorded zero citations for this code—a notable absence that suggests either improved compliance or lower inspection frequency for this specific violation.
When 393.83(d) citations do occur, they carry serious consequences. Our records show a 71.8% out-of-service rate, meaning inspectors placed the vehicle out of service nearly 7 out of 10 times this violation was discovered. That rate is substantially higher than the all-FMCSR average of 31.4%, indicating that inspectors view exhaust-system defects on non-gasoline buses as safety-critical.
Nationally, 393.83(d) ranks #1030 out of 3,036 FMCSR codes by citation volume. While the total citation count is relatively low, the high out-of-service rate underscores why this violation matters: when found, it typically stops your bus from continuing its route.
Who gets cited most
Our inspection records show this citation is concentrated in specific carrier types. San Diego Unified School District leads all carriers with 55 citations, followed by Los Angeles Unified School District with 17 citations and Tornado Bus Company with 12 citations. Our data shows fleets such as these have encountered this defect multiple times, though the absence of recent citations in our 12-month and 90-day windows suggests heightened awareness or remediation.
The carrier concentration reflects the reality that this code applies exclusively to non-gasoline-powered buses—predominantly school districts and transit operators that have made the operational choice to use diesel or alternative fuels for cost and emissions reasons.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.83(d) stands apart in both citation volume and severity. Consider these peer codes:
- 393.9(a) — Inoperable required lamps has been cited 660,737 times with a 15.4% out-of-service rate. While far more common, lamp defects trigger OOS enforcement at roughly one-fifth the rate of exhaust violations.
- 396.3(a)(1) — Inspection/repair/maintenance general accounts for 236,919 citations with a 45.3% out-of-service rate, placing it between lamps and exhaust in severity but ahead in citation frequency.
- 393.78 — Windshield condition defective has 157,894 citations but only a 0.3% out-of-service rate, making it much less likely to remove a vehicle from service despite high citation volume.
The 71.8% out-of-service rate for 393.83(d) puts it among the most serious maintenance violations enforced roadside. Inspectors treat exhaust defects as safety-critical because improper exhaust discharge poses environmental, noise, and potential vehicle control hazards.
How to avoid it
Exhaust-system integrity is a pre-trip responsibility. Before you drive, incorporate these actions into your inspection routine:
-
Walk around the entire bus during your pre-trip inspection. Locate the exhaust outlet and trace the exhaust line from the engine toward the rear. Look for any visible gaps, cracks, or holes in the pipe or muffler. If you see rust holes or corrosion, report it immediately to your dispatcher or maintenance.
-
Check that the exhaust pipe is firmly fastened. The system should not rattle, move side-to-side, or hang loose when you tap it gently. Exhaust clamps and hangers deteriorate over time, especially on older buses or those exposed to road salt. A loose exhaust pipe is an automatic citation and OOS order.
-
Confirm the exhaust outlet is clear and positioned correctly. The outlet should not be blocked by debris, ice, or mud. On buses, the exhaust should exit below or to the side of the passenger compartment, not upward or toward other vehicles. If you notice the outlet is clogged or redirected, don't ignore it—fuel efficiency and engine performance will suffer.
-
Listen for unusual exhaust noise during start-up. A loud rattling, hissing, or roaring sound when the engine first turns over is a red flag. It often signals a loose or damaged component inside the muffler or a crack in the manifold.
-
Report wear and damage to your maintenance team immediately. If you notice any of the above issues during a pre-trip, do not drive the bus. Contact your dispatcher and request roadside repair or tow service. A citation and OOS order is far more costly than preventive maintenance.
-
If your bus recently had engine work or service, ask your mechanic to confirm that all exhaust clamps, gaskets, and hangers were reinstalled correctly. Service shops occasionally miss these during repairs, especially if work was rushed.
The 71.8% out-of-service rate on this violation means that once cited, your bus will almost certainly be pulled from service and you will not complete your route that day. Prevention through diligent pre-trip inspection is your only reliable defense.