What 390.21(a) means in plain language
Every commercial motor vehicle on the road is required to display the legal name or USDOT number of the motor carrier that is operating it. The markings have to be visible from the outside, which means they need to appear on both sides of the power unit. If your truck is rolling without that information clearly posted, you are in violation of 390.21(a) — even if the rest of the vehicle is in perfect mechanical condition.
This is fundamentally an identification rule. Enforcement officers need to be able to connect a vehicle to a carrier during a roadside stop, a weigh station check, or a crash investigation. When the marking is missing, illegible, or refers to the wrong entity, the officer cannot verify who is responsible for that vehicle's safety record.
The fix sounds simple — put the right name and number on the door — but the violation keeps occurring because drivers are not always the ones who maintain the markings, and a decal can fade, peel, or get painted over during a repower or a lease change without anyone catching it before the next inspection.
What our enforcement data actually shows
Across our 13 million+ inspection records, 390.21(a) has generated 25,872 all-time citations, placing it at #108 out of 3,036 FMCSR codes by citation volume. That is a top-4% enforcement footprint — this is not a rarely-written violation.
The pace has been accelerating. Our inspection records show 9,685 citations in the last 12 months and 2,096 citations in just the last 90 days. Looking at the monthly trend, volume has consistently run between 799 and 968 citations per month from January through March 2026, with a peak of 968 in February 2026.
Here is the number that matters most if you are worried about being put out of service: the all-time OOS rate for 390.21(a) is effectively 0.0%. Only 3 of 25,872 citations resulted in an OOS order — that is 25,869 inspections where the driver kept rolling. For context, the all-FMCSR average OOS rate across all codes in our database is 31.4%. This violation sits at essentially zero. You will almost certainly be cited and released, not sidelined.
That said, the citation still lands in the CSA system with a severity weight of 3, which means it will affect your carrier's BASIC scores even though it will not ground the truck on the spot.
Who gets cited most
Looking at the last 180 days, Texas dominates the citation map with 4,470 citations and a 0.0% OOS rate. North Carolina is a distant second at 151 citations, also at 0.0% OOS. New Mexico recorded 18 citations with a 0.0% OOS rate over the same period. The OOS rate is consistent across all three states — there is no material variation — so the risk of being placed out of service is uniformly low regardless of where you are stopped.
Texas's volume is striking. At 4,470 citations in 180 days, it accounts for the vast majority of recent enforcement activity on this code. If you run cross-border routes through the Southwest, this is the state where officers are most actively writing 390.21(a).
On the carrier side, our data shows fleets such as LUCA AUTOTRANSPORTES SA DE CV (USDOT 3210458) with 47 all-time citations and REPUBLIC WASTE SERVICES OF TEXAS LTD (USDOT 772407) with 24 citations appearing at the top of the citation counts. The concentration of Mexican-domiciled carriers in the top-cited list is consistent with the heavy Texas enforcement volume, given the volume of cross-border commercial traffic through that corridor.
How severe is this compared to similar codes
390.21(a) sits alongside several peer codes in the General/Admin category, and comparing them puts your citation in context.
390.21TB2-DOT, which covers a related marking requirement for DOT numbers on trucks, has accumulated 74,663 citations — nearly three times the volume of 390.21(a) — at a 0.0% OOS rate. 390.21T(b), another marking-related sub-requirement, shows 61,097 citations, also at 0.0% OOS. 390.21TB1-MC, covering the motor carrier marking variant, has 59,189 citations at 0.0% OOS.
The pattern is consistent: every peer code in this marking family carries essentially zero OOS risk. None of them will park your truck. What they share is administrative consequence — CSA points accumulate, DataQs challenges become necessary, and carriers with repeated marking violations attract intervention attention. The volume across these related codes, which collectively total well over 200,000 citations in our database, tells you that marking compliance is an active enforcement priority, not a checkbox officers overlook.
How to avoid it
The co-occurring violation data from our last 90 days gives a clear picture of what kind of inspections produce 390.21(a) citations — and what you should be checking before you pull out.
- Verify your markings at every pre-trip. Walk both sides of the cab and confirm the carrier's legal name or USDOT number is present, legible, and at least 2 inches tall. A faded or peeling decal is all it takes.
- Check after any repaint, wrap, or body repair. Our data shows the violation frequently appears alongside other basic compliance gaps. If bodywork was done, confirm the markings were reapplied correctly before the truck goes back into service.
- Know your carrier's current legal name. If the company has changed its name, been acquired, or if you are leased to a different carrier than when the markings were applied, what is on the door may no longer match what is in FMCSA's system. Verify the USDOT number on the door matches the operating carrier.
- Address lighting and equipment deficiencies at the same time. In our last 90 days of data, 390.21(a) appeared alongside 393.9 (Inoperable Required Lamp) in 793 shared inspections and alongside 393.11 (Lighting devices/reflectors) in 270 shared inspections. A thorough pre-trip that catches lamp and lighting defects will often catch the conditions that lead to the full vehicle inspection where marking violations get written.
- Carry your medical certificate and verify CDL status. Our records show 391.41APC (medical certificate possession) co-occurring in 283 shared inspections and 383.23A2 (operating without a CDL) appearing in 343 shared inspections alongside 390.21(a). An officer who stops you for a marking violation and then finds document deficiencies will write everything they find.
- Pay attention if you drive a Freightliner or Kenworth. FRHT vehicles account for 4,243 all-time citations under this code, and KW units account for 2,393. These are high-volume platforms — make sure markings are part of the standard pre-trip checklist for these makes in your fleet.