What 393.55B means in plain language
A 393.55B citation means an inspector found that your coupling device or towing method is defective or inadequate. Your coupling device is the mechanical connection between your tractor and trailer—the kingpin assembly, the fifth wheel, or whatever hardware locks your units together. If that connection is worn, cracked, bent, missing fasteners, or otherwise unable to reliably hold the load, you get cited.
This isn't about minor wear. Inspectors are looking for defects that pose a genuine safety risk: a fifth wheel with a bent or broken locking mechanism, a kingpin that's been twisted or sheared, rust or corrosion that's compromised structural integrity, or improper setup that allows the trailer to shift. The regulation requires that coupling hardware be functional and strong enough to keep your trailer securely attached during normal operation.
Unlike some vehicle maintenance violations, 393.55B is never placed out of service at roadside. That means the inspector is citing you for a defect but allowing you to drive. However, you should address it before your next inspection cycle.
What our enforcement data actually shows
Across our 13 million+ inspection records, 393.55B has received 301 all-time citations, with 138 citations in the last 12 months and 41 in the last 90 days. It ranks #1086 out of 3,036 FMCSR codes by citation volume—a low-frequency violation overall.
What's most striking: our data shows a 0.0% out-of-service rate for 393.55B. None of the 301 inspections resulted in an out-of-service placement. For context, the average FMCSR code carries a 31.4% out-of-service rate. This tells you that when inspectors cite 393.55B, they typically see the defect as correctable without immediate roadside removal—though you should still treat it as urgent.
The 41 citations in the last 90 days represent a slight uptick. Looking at the 12-month trend, citations peaked in March 2026 at 20, and July 2025 at 16, suggesting seasonal or enforcement-cycle variation.
Who gets cited most
Our inspection records show that 393.55B citations concentrate heavily in Texas, which accounts for 53 citations over the last 180 days—more than half of all citations in that period. Illinois ranks second with 10, and North Carolina third with 3. All three states show a 0.0% out-of-service rate, consistent with the national pattern.
Among carriers, our data shows Federal Express Corporation (USDOT 86876) with 7 citations all-time, the highest count in our database. This does not imply systemic negligence; large carriers with more vehicles naturally accumulate more citations. Other fleets such as Potros Concrete LLC, Smith Empire LLC, and Mega Discounts LLC each appear with 2 citations, reflecting the scattered nature of this violation across the industry.
How severe is this compared to similar codes
393.55B sits in the Vehicle Maintenance category alongside codes like 393.9 (Inoperable required lamps) and 393.78 (Windshield condition defective). To put its enforcement frequency in perspective: 393.9 has generated 660,737 all-time citations with a 15.4% out-of-service rate; 393.78 has 157,894 citations with a 0.3% OOS rate. By comparison, 393.55B's 301 citations make it a rare finding.
Another relevant peer is 393.47E (Slack adjuster defective), which has 180,363 citations but also a 0.0% out-of-service rate. This pattern—a brake or coupling defect, high citation count elsewhere, zero OOS—suggests that many coupling and brake defects are treated as correctable warnings rather than immediate safety removals.
393.55B's CSA severity weight is 8, a moderate value that reflects concern without the highest alarm level.
How to avoid it
Based on the patterns in our inspection data, here are concrete steps to prevent a 393.55B citation:
-
Inspect your fifth wheel or kingpin assembly during every pre-trip. Look for visible cracks, bent metal, rust, or missing bolts. Ensure the locking mechanism engages fully and the pin/plate interface is clean and tight. Our data shows coupling defects often cluster with inoperable lamps (393.9 co-occurred 17 times in the last 90 days) and emergency equipment issues (15 co-occurrences with 393.95A), suggesting inspectors scrutinize the entire coupling system when they stop your rig.
-
Keep fasteners tight. Fifth wheel bolts work loose over time, especially on Freightliners (61 citations in our data) and Kenworths (34 citations). Carry a wrench and check bolts weekly or per your carrier's schedule.
-
Clean the fifth wheel plate and kingpin. Mud, grease, or corrosion reduces the strength of the connection. Wash these components monthly and apply appropriate lubricant per manufacturer specs.
-
Monitor for wear and corrosion. If you notice the fifth wheel or kingpin showing rust, pitting, or deformation, report it to maintenance immediately. Don't assume minor cosmetic rust is safe; inspectors may cite it if it's structural.
-
Check the trailer connection before departure. Confirm the trailer is fully seated and locked. A partially engaged kingpin or fifth wheel that hasn't fully engaged is a primary defect cited under 393.55B.
-
Report maintenance issues early. Our data shows 393.55B often appears alongside 396.17C (no proof of periodic inspection) and 383.23A2 (operating without a CDL), suggesting that compliance attention varies. Build a habit of documenting pre-trip findings and communicating defects to your fleet's maintenance team before the defect becomes an inspection liability.
Since 393.55B is not an out-of-service violation, you have time to correct it—but don't delay. A second citation in the same fiscal year can elevate your carrier's safety profile and increase inspection frequency.