What 178.337-9C means in plain language
FMCSR 178.337-9C governs the proper marking of inlets and outlets on MC331 cargo tanks — the pressurized containers used to transport certain hazardous materials. The regulation requires that these openings be clearly and correctly identified so that loaders, unloaders, and emergency responders can quickly understand what each connection does and what material flows through it.
This isn't about having a placard on the side of your truck. It's about labeling the specific access points on the tank itself — the fill ports, discharge connections, and venting systems. When an inspector cites you for this code, they've found that one or more of these inlets or outlets either lacks proper marking entirely, or the marking is illegible, incorrect, or doesn't match the hazmat being transported.
For drivers, this typically surfaces during pre-trip inspections or at loading/unloading facilities. A missing or smudged label on a tank connection is a straightforward fix, but it's one that matters for safety and compliance.
What our enforcement data actually shows
Across our 13 million+ inspection records, 178.337-9C is exceptionally rare. We've recorded just 3 all-time citations for this violation, with 0 citations in the last 12 months and 0 in the last 90 days. This code ranks #2551 out of 3,036 FMCSR codes by citation volume.
None of the 3 citations resulted in out-of-service placement, giving this code a 0.0% OOS rate. By contrast, the all-FMCSR average OOS rate is 31.4%, meaning inspectors almost never deem a marking inlet/outlet violation serious enough to pull a vehicle off the road immediately. This suggests that when citations do occur, they're treated as correctable defects rather than safety-critical failures.
The absence of any citations in the past 90 days and 12 months indicates that this violation has largely disappeared from roadside inspection activity in the current enforcement environment.
Who gets cited most
Our data is too sparse to identify state patterns — we have only 3 all-time citations, so geographic clustering is not meaningful. However, our records do show that carriers such as California Gas Transport Inc (USDOT 589266), High Pressure Transports LLC (USDOT 1658917), and Impetus LLC (USDOT 3058664) each had 1 citation. This reflects the extreme rarity of the violation across the industry rather than a concentration of risk in any particular fleet.
How severe is this compared to similar codes
MC331 marking sits within the broader hazardous materials category, alongside placarding and loading/unloading violations. The contrast is striking. Our data shows that general loading and unloading hazmat violations (177.834A-HMC and 177.834(a)) generate thousands of citations — 3,954 and 3,839 respectively — and carry OOS rates of 99.2% and 97.9%. Placarding violations (177.817(a)) account for 2,274 citations with a 75.1% OOS rate.
By comparison, 178.337-9C's 3 citations and 0.0% OOS rate indicate that marking inlets and outlets is enforced far less aggressively than broader hazmat compliance issues, and violations are not treated as immediately dangerous. Even among peer codes like 172.502(a)(1) (placarding general requirements) with 1,820 citations and an 18.5% OOS rate, this marking code is vastly less cited and less likely to result in roadside removal.
How to avoid it
Because this violation is so uncommon, it typically doesn't emerge as a pattern in co-occurring defects. However, drivers and fleet managers can take straightforward steps to ensure full compliance:
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Inspect tank connections during pre-trip. Before accepting or operating an MC331 tank, verify that every inlet, outlet, and connection point — fill ports, discharge valves, venting systems — is clearly and legibly marked with the correct identification.
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Check markings match the load. Confirm that the labels on inlets and outlets accurately reflect the hazmat being transported. Mismatched or outdated markings create confusion and violate the regulation.
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Keep markings clean and visible. During your walkaround, wipe down tank connections if dirt or grime obscures labels. A marking that can't be read is as problematic as a missing one.
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Report missing or damaged labels immediately. If you discover a blank or illegible inlet/outlet marking during pre-trip, notify your dispatcher or the facility operator before loading or unloading begins. Do not operate the vehicle until corrections are made.
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Document tank condition. Take photos of marked inlets and outlets before and after loading. This creates a record if a citation later arises and helps your fleet track whether a defect was pre-existing or developed during your haul.
Because citations for this violation are so rare — and because none result in out-of-service placement — the real risk is operational delay or facility rejection. A conscientious pre-trip walk-around and basic maintenance of tank labels will keep you compliant and moving.