What 393.9B-LSLO means in plain language
Your stop lamps are the red lights at the rear of your truck that illuminate when you brake. FMCSR 393.9B-LSLO is cited when an officer finds one or more of these lamps obscured—meaning blocked, covered, or otherwise hidden from view. This includes damage (cracks or cloudiness that reduces visibility), dirt, mud, snow, or anything else that prevents the light from shining clearly to traffic behind you.
Obscured stop lamps create a safety hazard. Drivers behind you need to see your brake lights instantly to avoid rear-end collisions, especially in low-light or poor-weather conditions. An inspector at the roadside will check both sides of your truck and determine whether the lamps are clearly visible. If even one is obscured enough that light output is compromised, you can be cited.
This is a maintenance issue, not a driver behavior violation. It means your truck's lighting system needs attention before it goes back on the road—but our data shows this citation is usually resolved without placing the vehicle out of service.
What our enforcement data actually shows
Across 13 million roadside inspections, we recorded 638 all-time citations for 393.9B-LSLO, with 393 citations issued in the last 12 months and 89 in the last 90 days. This code ranks #848 out of 3,036 FMCSR codes by citation volume, placing it in the lower half for enforcement frequency.
The out-of-service (OOS) rate for this violation is 2.5%—meaning only 16 of 622 cited trucks were placed out of service. This is substantially lower than the all-FMCSR average OOS rate of 31.4%, indicating that obscured stop lamps are rarely severe enough to warrant immediate removal from service. Inspectors typically cite you and expect correction before your next departure or within a defined window.
Enforcement has been trending upward recently. In the 12-month period we track, monthly citations ranged from 13 (April 2025) to 46 (January 2026 and August 2025), showing seasonal and operational variation but no dramatic spike.
Who gets cited most
Our inspection records show geographic clustering. California leads with 20 citations in the last 180 days, followed by Pennsylvania with 19, and Florida with 11. However, the OOS rates vary significantly: California's 15.0% OOS rate is substantially higher than Pennsylvania's 5.3%, while Florida recorded zero out-of-service placements across 11 citations. This suggests California inspectors may apply a stricter threshold for when obscured lamps warrant removal from service.
Kansas, Arizona, Georgia, New York, Wisconsin, Missouri, and Colorado each recorded 6–10 citations in the same period, all with 0% OOS rates. The pattern indicates this violation is cited nationwide but with regional variation in enforcement severity.
Among carriers in our database, Auto Haul Express LLC (USDOT 4329325) and Hugill Sanitation Inc (USDOT 2114510) each appear with multiple citations, though overall citation counts across carriers remain low, suggesting the issue is not concentrated in specific large fleets.
How severe is this compared to similar codes
In the vehicle maintenance category, 393.9B-LSLO sits firmly on the lower-severity end. For comparison, inoperable required lamps (393.9(a)) generated 660,737 citations with a 15.4% OOS rate—far higher citation volume and OOS frequency. The related code 393.9 (inoperable required lamp, a broader category) shows 180,097 citations and 6.9% OOS rate, still well above obscured lamps.
Lighting devices/reflectors generally (393.11) produced 179,734 citations but only a 1.8% OOS rate, closer to the obscured-lamp profile. Meanwhile, codes like inspection/repair/maintenance general (396.3(a)(1)) carry a 45.3% OOS rate, indicating those violations are treated as much more serious. Our data shows that obscured lamps—being cosmetic or easily corrected—are among the most lenient lighting citations.
How to avoid it
Stop lamp obscuration is preventable with routine pre-trip inspections:
- Walk around your truck before every shift. Check both rear corners and the center high-mounted brake light. Look for dirt, mud, snow, condensation, or damage to the lens.
- Clean your lights weekly or more in wet/muddy conditions. Use a soft cloth and mild cleaner. Do not use abrasive pads that scratch the lens.
- Inspect the lens for cracks or cloudiness. If the plastic is cracked, discolored, or milky, replace it. Even small damage can diffuse light and trigger a citation.
- Test your brakes before departing. Have a spotter watch your rear lights while you tap the pedal, or use a wall/loading dock door. If a light doesn't illuminate or appears dim, investigate before leaving.
- Check for corrosion around lamp housings. Road salt and moisture can corrode contacts. If a lamp flickers or appears intermittent, clean the connection or replace the bulb.
- Ensure mudflaps or bumpers are not blocking rear lights. Verify clearance and alignment after any repairs or attachment changes.
Our data shows that obscured-lamp citations often co-occur with other lighting violations—turn signal obscuration (393.9B-LTSO) appeared in 23 shared inspections in the last 90 days—so a thorough lighting walk-around catches multiple issues at once. Maintenance of all rear lighting is worth 10 minutes of your time and avoids both a citation and a crash.