What 178.337-9(c) means in plain language
FMCSR 178.337-9(c) requires that inlets and outlets on MC331 cargo tanks be properly marked. An MC331 is a specific type of pressure vessel used to transport certain hazardous materials—typically liquefied gases or other pressurized cargo.
The regulation exists because shippers, loaders, carriers, and emergency responders need to instantly identify where product enters and exits the tank. A missing or illegible marking on an inlet or outlet can lead to loading errors, incompatible products being mixed, or dangerous delays during an emergency response.
You'll see this violation cited when an inspector finds that the physical markings identifying inlet and outlet locations are absent, worn away, or obscured. It's a documentation and visibility issue on the equipment itself, not about how you handle the load or drive the truck.
What our enforcement data actually shows
Across 13 million inspections in our database, 178.337-9(c) has generated only 49 all-time citations—ranking #1640 out of 3,036 FMCSR codes by citation volume. In the last 12 months, we recorded zero citations for this violation, and zero in the last 90 days.
None of the 49 citations on record resulted in an out-of-service order. The out-of-service rate for this code is 0.0%. By comparison, the all-FMCSR average OOS rate is 31.4%, meaning 178.337-9(c) is extremely unlikely to remove your truck from service, even if cited.
The rarity and non-OOS nature of this violation reflect that it's typically a minor equipment marking defect rather than an acute safety hazard. Inspectors catch it when performing detailed hazmat cargo tank inspections, but it almost never triggers immediate roadside enforcement action that takes a vehicle out of operation.
Who gets cited most
Our inspection records show this violation is scattered across the trucking population. Windstar Trucking LLC (USDOT 2126272) accounts for 3 citations—the highest single carrier count. The remaining citations are distributed across eight other carriers with one citation each, including Texas International Gas & Oil Company, Valley Wide Cooperative Inc, Blossman Gas Inc, Central United Cooperative, United Petroleum Transports Inc, Ryan Bros Trucking Inc, Jerry's U-Save Inc, and Light Gas Corporation.
This spread indicates that 178.337-9(c) violations are not concentrated in any single fleet or operation type. They occur sporadically across different carrier sizes and business models, suggesting the issue is typically a maintenance or equipment condition issue rather than a systemic compliance failure.
How severe is this compared to similar codes
Hazardous materials codes in the same family show a much different enforcement picture. The peer code 177.834A-HMC (General loading/unloading hazmat) has 3,954 citations with a 99.2% out-of-service rate. Similarly, 177.834(a) has 3,839 citations and a 97.9% OOS rate. Even 177.817(a) (Placarding violation) generated 2,274 citations at a 75.1% OOS rate.
By contrast, 172.516(c)(6) (Placard damaged deteriorated or obscured) mirrors 178.337-9(c)'s low-OOS pattern with 1,796 citations and only 1.6% OOS rate. And 172.602(c)(1) (Maintenance/accessibility of Emergency Response information) shows 1,464 citations with a 0.0% OOS rate—identical to what we see here.
This code sits in a category of hazmat marking and visibility issues that rarely result in vehicle removals, even though they're critical for safety documentation.
How to avoid it
Inlet and outlet markings on an MC331 cargo tank are fixed equipment that you or your fleet should inspect and maintain regularly:
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During pre-trip: Walk around the entire cargo tank (or have a qualified mechanic do so monthly). Look for inlet and outlet ports and verify any stenciled or stamped markings are fully visible and legible from normal viewing distance. Use a flashlight if necessary and check for rust, paint wear, or dirt obscuring the markings.
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Document tank condition: Keep a simple log or photos of tank markings during routine maintenance cycles. If markings are fading, plan a touch-up before an inspection catches it.
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Know your equipment: If you operate or own an MC331, understand which ports are inlets and which are outlets for your specific tank design. The marking scheme should be consistent with the tank manufacturer's documentation.
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Report to maintenance: If you notice a marking is missing or illegible during a pre-trip walk-around, report it to your fleet's maintenance department immediately. Do not depart with unmarked or illegible ports.
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Coordinate with dispatch: If you haul hazmat in pressurized tanks, confirm before loading that tank markings are current. Some fleets require a photo or checklist sign-off before accepting a loaded tank at a shipper facility.
Because this violation is rare and non-OOS, it's usually corrected with a maintenance repair order rather than a citation. Staying proactive about equipment visibility keeps your compliance record clean and ensures emergency responders can identify your cargo safely if needed.